bug: Workspace chat readability — tight line spacing, narrow message column, scrollbar in the empty composer

[vybe]

Summary

Three readability defects in the Workspace chat surface (/workspace), all in src/frontend/src/components/portal/PortalConversation.vue (and its twin PortalRoom.vue): message lines sit too tightly together, the message column uses only a narrow slice of the available panel width, and the composer renders a vertical scrollbar while it is empty.

Context

Reported from live use of the Workspace. The text reads "stuffy" — dense line spacing plus a narrow column — and the empty composer shows a scrollbar it has nothing to scroll.

1. Line spacing is too tight. The thread is space-y-4 with bubbles at px-3.5 py-2 text-sm, and the assistant's rendered markdown is squeezed further by the scoped rule .prose-portal :deep(p) { margin: 0.25rem 0; } — 4px between paragraphs, at the default text-sm leading. Needs a modest increase in line-height and inter-message/inter-paragraph rhythm (a tiny bit larger, not a redesign).

2. Message column is too narrow. Both the thread and the composer are capped at max-w-3xl mx-auto (48rem), and bubbles cap again at max-w-[85%] of that. On a wide viewport most of the panel is empty gutter. The column should take a higher percentage of the available panel width while staying readable.

3. Empty composer shows a vertical scrollbar. autoGrow() does:

el.style.height = 'auto'
el.style.height = Math.min(el.scrollHeight, 160) + 'px'

The textarea is box-sizing: border-box with a 1px border on each side (Tailwind border), and scrollHeight excludes borders — so the applied height is 2px short of the single line it must hold, and the browser shows a scrollbar even at zero characters. overflow-y is also never toggled to hidden below the max-h-40 ceiling. Reproduces after any input event that leaves the field empty (send, select-all-delete). The identical composer + autoGrow() exists in PortalRoom.vue, so any fix must land in both.

Acceptance Criteria

• Chat message line spacing is visibly looser (line-height and inter-message/paragraph rhythm), without changing the bubble language or font size
• The message/composer column takes a larger share of the available panel width on wide viewports, with a sensible readable ceiling; narrow/mobile layout unchanged
• An empty composer never shows a vertical scrollbar; the scrollbar appears only once content exceeds the max-h-40 ceiling
• The composer fix is applied to both PortalConversation.vue and PortalRoom.vue (same code, two files)
• Changes use semantic tokens only per docs/memory/design-system-contract.md — no raw Tailwind palette classes, no hardcoded hex; verified in light and dark
• No layout shift or scroll-position reset introduced in the thread on send/receive

Technical Notes

• Files: src/frontend/src/components/portal/PortalConversation.vue (thread max-w-3xl + space-y-4, composer textarea, autoGrow(), scoped .prose-portal block), src/frontend/src/components/portal/PortalRoom.vue (same composer and autoGrow())
• Scrollbar root cause: under box-sizing: border-box, scrollHeight omits border width — the assigned height needs the border box accounted for (or the textarea needs overflow-y managed explicitly below the ceiling)
• Read docs/memory/design-system-contract.md before touching any file under src/frontend/
• Raw-color ratchet: node src/frontend/scripts/scan-raw-colors.mjs src/frontend must not regress per-file counts