docs: design template catalog

docs/template-catalog.md

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+# Template Catalog Browsing Experience
+
+Design specification for the template discovery flow at `/app/templates`.
+
+---
+
+## Layout
+
+```
+┌─────────────────────────────────────────────────────────┐
+│  Template Catalog                                        │
+│  Browse and select from our collection of templates.     │
+├─────────────────────────────────────────────────────────┤
+│  [🔍 Search templates…]   [All][DEX][Lending][Payment]  │
+│                           [Asset Issuance]               │
+├─────────────────────────────────────────────────────────┤
+│  4 templates found                                       │
+│                                                          │
+│  ┌──────────┐  ┌──────────┐  ┌──────────┐              │
+│  │ Card     │  │ Card     │  │ Card     │              │
+│  └──────────┘  └──────────┘  └──────────┘              │
+└─────────────────────────────────────────────────────────┘
+```
+
+---
+
+## Filter Controls
+
+### Search
+
+- Placement: top-left of the filter bar, `max-w-md`, full-width on mobile.
+- Input type: `type="search"` with a leading magnifying-glass icon.
+- `aria-label="Search templates"`, `placeholder="Search templates…"`.
+- Debounce: 300 ms before the query is applied and the URL is updated.
+- Matches against template `name` and `description`.
+
+### Category Chips
+
+- Placement: right of the search input on ≥ sm screens; wraps below on mobile.
+- Options: **All** (clears filter) · **DEX** · **Lending** · **Payment** · **Asset Issuance**.
+- Each chip is a `<button>` with `aria-pressed` reflecting active state.
+- Active chip: `primary-gradient` background, `text-on-primary`.
+- Inactive chip: `bg-surface-container-low`, `border border-outline-variant/20`.
+- Clicking an already-active category chip deselects it (returns to All).
+- The group carries `role="group" aria-label="Filter by category"`.
+
+### Sort
+
+Sort is not exposed in the current UI. Templates are returned in server-defined order (active templates, seeded via migration). A sort control may be added in a future iteration.
+
+### URL Persistence
+
+Active filters are serialised into the URL query string (`?category=dex&search=stellar`) so that the state survives page reload and can be shared via link.
+
+---
+
+## Card Anatomy
+
+```
+┌────────────────────────────────┐
+│  [Preview image / icon]        │  ← aspect-video, lazy-loaded
+│  ┌─ Category badge ─┐          │  ← top-left overlay, primary bg
+│  └──────────────────┘          │
+├────────────────────────────────┤
+│  Template Name                 │  ← font-headline, font-bold
+│  Short description (2 lines)   │  ← line-clamp-2, text-on-surface-variant
+│                                │
+│  ─────────────────────────── │
+│  N features enabled  [Use →]   │  ← feature count + CTA button
+└────────────────────────────────┘
+```
+
+| Element | Detail |
+|---|---|
+| Preview area | `aspect-video`; shows `previewImageUrl` if set, otherwise a category emoji fallback |
+| Category badge | Absolute-positioned top-left; label from `CATEGORY_LABELS` map |
+| Name | `<h3>`, `font-headline font-bold text-on-surface`; hover transitions to `text-on-primary-container` |
+| Description | `text-sm text-on-surface-variant`, `line-clamp-2` |
+| Feature count | Count of `features` where `enabled === true`; pluralised |
+| CTA | `<button>` "Use Template" with chevron icon; `aria-label="Use {name} template"` |
+
+The card is an `<article>` with hover shadow and border transition (`hover:shadow-lg hover:border-outline-variant/25`).
+
+---
+
+## States
+
+### Loading
+
+- Shown while the API request is in flight.
+- Renders 6 skeleton cards in the same 3-column grid.
+- Each skeleton mirrors the card structure with `animate-pulse` placeholder blocks.
+- The grid has `role="status" aria-label="Loading templates"` and a visually-hidden `"Loading templates…"` text for screen readers.
+
+### Error
+
+- Shown when the fetch fails (network error, non-2xx response).
+- Uses the shared `<ErrorState>` component: title "Failed to load templates", the error message, and a "Try Again" button.
+- Retry re-issues the same request with the current filters.
+- Special case: HTTP 429 surfaces "Too many requests. Please try again in a moment."
+
+### No Results (filtered)
+
+- Shown when filters are active but the API returns an empty array.
+- Uses `<EmptyState>` with icon `🔍`, title "No templates found", description "Try adjusting your search or filters…", and a "Clear Filters" action button.
+
+### No Results (unfiltered)
+
+- Shown when no filters are active and the API returns an empty array.
+- Uses `<EmptyState>` with icon `📋`, title "No templates available", description "Templates will appear here once they are published." No action button.
+
+### Results count
+
+- When filters are active and results exist, a live region (`aria-live="polite"`) above the grid reads "{N} template(s) found".
+
+---
+
+## Mobile Behavior
+
+| Breakpoint | Behavior |
+|---|---|
+| `< sm` (< 640 px) | Search input is full-width; category chips wrap to a second row; grid is 1 column |
+| `sm – md` (640–1024 px) | Search and chips sit on one row (`flex-row`); grid is 2 columns |
+| `≥ lg` (≥ 1024 px) | Full layout; grid is 3 columns |
+
+The filter bar uses `flex flex-col gap-4 sm:flex-row sm:items-center sm:justify-between` so it reflows naturally. Category chips use `flex flex-wrap gap-2` to prevent overflow on narrow screens.
+
+---
+
+## Category Discovery
+
+- All four categories are always visible as chips — no hidden dropdowns.
+- The **All** chip is selected by default, showing the full catalog.
+- Selecting a category immediately filters the grid (no submit step).
+- The active category is reflected in the URL and the chip's `aria-pressed` state.
+- Category labels and emoji icons are defined in `CATEGORY_LABELS` / `CATEGORY_ICONS` maps inside `TemplateCard.tsx` and `TemplateCatalogFilters.tsx`.
+
+### Categories
+
+| Value | Label | Icon | Use case |
+|---|---|---|---|
+| `dex` | DEX | 📊 | Decentralized exchange for trading Stellar assets |
+| `lending` | Lending | 🏦 | Lending / borrowing protocols |
+| `payment` | Payment | 💳 | Payment gateway and invoice management |
+| `asset-issuance` | Asset Issuance | 🪙 | Custom Stellar asset creation and distribution |
+
+---
+
+## Accessibility
+
+- Filter group: `role="group" aria-label="Filter by category"`.
+- Category chips: `aria-pressed` boolean reflects selection state.
+- Search: `type="search"`, `aria-label="Search templates"`.
+- Loading skeleton: `role="status"` with visually-hidden label.
+- Results count: `aria-live="polite"` region.
+- Card CTA: `aria-label="Use {template.name} template"` for unique button labels.
+- Preview images: `alt="{name} preview"`; emoji fallback is `aria-hidden="true"`.
+
+---
+
+## Implementation Reference
+
+| File | Purpose |
+|---|---|
+| `apps/frontend/src/app/app/templates/page.tsx` | Page — wires filters, URL sync, and routing |
+| `apps/frontend/src/components/app/templates/TemplateCatalogFilters.tsx` | Search input + category chips |
+| `apps/frontend/src/components/app/templates/TemplateGrid.tsx` | Grid, loading skeleton, error, and empty states |
+| `apps/frontend/src/components/app/templates/TemplateCard.tsx` | Individual template card |
+| `apps/frontend/src/components/app/templates/useTemplates.ts` | Data-fetching hook with filter state and URL helpers |
+| `packages/types/src/template.ts` | `Template`, `TemplateCategory`, `TemplateFilters` types |
+
+---
+
+## Edge Cases & Assumptions
+
+- **Slow network**: the loading skeleton prevents layout shift; the abort controller cancels in-flight requests when filters change rapidly.
+- **No preview image**: the category emoji is shown as a fallback — no broken-image state.
+- **Long template names**: the card title does not clamp; descriptions clamp at 2 lines.
+- **Single result**: the results count reads "1 template found" (singular).
+- **Sort**: not implemented; order is determined server-side. A future sort control (e.g. "Newest", "Most deployed") can be added to the filter bar without breaking the existing layout.
+- **Pagination**: not implemented in the current UI; the API supports `limit`/`offset` for future use.