refactor(coc/spa): redesign chats list with status-priority groups + mode pills

[plusplusoneplusplus]

Summary

Redesigns the Activity / Chats list pane to match the OpenDesign reference (status-priority grouping, compact rows, mode pills, styled search). UI-only — no functional behavior changes.

• Group rows by Running → Pinned → Today → This week → Older (plus collapsible Archived), with sticky tinted section headers and counts.
• New compact row: status dot (pulses blue running / red failed / grey done) + colored mode pill (ASK amber / PLAN blue / AUTO green) + title + relative time.
• Styled search input with magnifier icon and platform-aware ⌘F / Ctrl+F kbd hint; auto-hiding filter chips (All / Running / Failed) with live counts.
• All existing behavior preserved: pin, archive, multi-select, context menu, draft badges, unseen dots, session-category badges, FTS5 server search, load-more, Ctrl+F shortcut (now also focuses the chats search input via the existing searchInputRef).
• Existing data-testid hooks kept; updated two source-string tests in ChatListPane.test.ts that pinned the prior structure.

Test plan

• vitest run for ChatListPane* — 454/454 pass.
• Full SPA test suite — 14,581/14,583 pass; remaining 2 failures are pre-existing and depend on a built bundle.js artifact unrelated to this change.
• Visual smoke check in the dashboard (Chats tab): groups render, mode pills colored, status dot animates while running, ⌘F focuses search, filter chips toggle, archived section collapses.
• Verify pin / archive / unarchive / context-menu / multi-select still work via the new rows.
• FTS5 search results render with snippet highlights and clickable rows.

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