feat: CMD+F find-in-channel search

[wesbillman]

Summary

• Adds CMD+F (Ctrl+F) find-in-channel search with a find bar UI, match highlighting, and prev/next navigation
• Two-tier search: instant client-side substring matching + debounced relay-backed Typesense query scoped to the channel via new channel_id filter
• Highlights matches inline using a rehype plugin that wraps matches in <mark> elements with accent-colored background (bg-primary/20)
• Keyboard navigation: Enter/Shift+Enter for next/prev, Escape to close

Key files (16 changed, 520 insertions)

• ChannelFindBar.tsx — find bar UI (input, match counter, prev/next/close)
• useChannelFind.ts — hook: state, two-tier matching, keyboard shortcut, channel-switch cleanup
• rehypeSearchHighlight.ts — rehype plugin wrapping matches in <mark> during HAST phase
• search.rs (relay) — channel_id filter with ACL validation
• messages.rs / models.rs (Tauri) — pass channel_id through to relay
• globals.css — reset native <mark> yellow background so accent color shows through

Test plan

• Open any channel, press CMD+F — find bar appears
• Type a query (≥2 chars) — matches highlight in accent color (not yellow)
• Press Enter / Shift+Enter to navigate between matches
• Press Escape to close and clear highlights
• Switch channels — find state resets cleanly
• Verify dark mode uses slightly stronger accent tint (bg-primary/30)

🤖 Generated with Claude Code

[wesbillman]