feat: add pin groups for organizing pinned items

[zyuanyuz]

✨ Pin Groups — Organize Pinned Items into Collapsible Folders

Summary

This PR adds pin groups to Maccy, allowing users to organize their pinned clipboard items into named, collapsible folders. A global preference controls whether groups start expanded or collapsed.

Motivation

Pinned items accumulate over time. Without grouping, the pinned section becomes a long flat list that is hard to navigate. Groups bring structure — users can categorize pins by project, type, or any scheme that fits their workflow.

Features

Group Management

• Create groups in Preferences → Pins → Groups section
• Rename or delete groups (deleting a group moves its items to ungrouped — pins are preserved)
• Groups are sorted by creation order with visual folder icons and item counts

Pinning Flow

• When pinning an item (⌥P), a group picker overlay appears
• Choose an existing group, create a new one on the spot, or leave ungrouped
• The group picker is skipped when unpinning (items are removed directly)

Popup Display

• Single group: items display inline without a header (backward compatible)
• Multiple groups: each group gets a collapsible header — click to expand/collapse
• Ungrouped pins display without a header below grouped ones
• Group sort order is respected within the pinned section

Global Preference

• "Expand groups by default" toggle in Preferences → Pins
• When enabled (default): all groups start expanded
• When disabled: all groups start collapsed, click to expand individually
• Toggling the preference updates all groups in real time — no restart needed

Data Model

Model	Fields
PinGroup	name, sortOrder, createdAt, items (relationship)
HistoryItem	pinGroup (optional relationship), pin (unchanged)

Code Changes

Area	Files	Purpose
Model	PinGroup.swift, HistoryItem.swift, Storage.xcdatamodel	PinGroup entity + relationship
Manager	PinGroupsManager.swift	CRUD, expand/collapse state, Defaults observer
Views	PinGroupHeaderView.swift, PinGroupPickerView.swift, PinsView.swift, ContentView.swift	UI for headers, picker overlay, grouped rendering
Settings	PinsSettingsPane.swift	Group management UI + expand preference toggle
Sorting	Sorter.swift	Group-aware sort priority
Config	Defaults.Keys+Names.swift	expandPinGroupsByDefault key
i18n	PinsSettings.strings (41 languages), PinGroups.strings (41 languages)	Full localization
Docs	README.md	Feature entries in Features + Usage sections

Backward Compatibility

• No migration needed: all existing pinned items have pinGroup = nil and display as "ungrouped"
• No breaking changes: the pin/unpin keyboard shortcuts work as before
• Single-group display has no header, matching the pre-group appearance

Screenshots

Group picker when pinning:

Collapsible group headers in popup:
Group management in Preferences: