Wire scrollback search UI to the FTS5 index

[buggerman]

Summary

• Final piece of scrollback search: Cmd+Shift+F now queries the FTS5 index from Add SQLite FTS5 scrollback index (no UI yet) #29 instead of doing an in-memory substring scan over currently-loaded messages. Searches every persisted message, not just what happens to be in @Observable model state right now.
• Adds a segmented scope picker (All servers / This server / This channel) so a user investigating one channel doesn't drown in cross-network noise.
• Surfaces FTS5's full query syntax — phrase ("foo bar"), prefix (bar*), boolean (term1 AND term2 NOT term3), column-scoped (sender:alice) — by passing the input through verbatim. The text-field placeholder advertises the syntax so users discover it without docs.

Changes

• Sources/Brygga/Views/ContentView.swift — GlobalFindSheet rewritten end-to-end:
  • Removes the GlobalFindMatch substring-scan plumbing. Holds @State private var hits: [SearchHit] instead, populated via .task(id:) that re-runs on every query or scope change with a 120ms debounce.
  • New ScopeUI enum + segmented Picker. The This server / This channel segments are disabled when there's no matching selection, so the picker can't silently downgrade behavior.
  • New SearchHitRow (private) replaces GlobalFindMatchRow. Renders serverName / target · timestamp and a 2-line hit.content excerpt.
  • open(_:) resolves (serverID, target) back to a channel for navigation, with sensible fallbacks: server-console hits land on the server row; hits whose target was closed since indexing land on the parent server row rather than no-op'ing.
  • Title changed from "Find in All Channels" to "Search Scrollback" — accurate now that scope is configurable and depth includes channels not currently open.

Test plan

• swift build — passes
• swift test — 131 tests pass (no test changes; UI is exercised by build + manual)
• swiftformat --lint . — clean
• Manual:
  • Cmd+Shift+F opens the new sheet. Default scope is "All servers".
  • Type a single word; results appear within ~120ms across every persisted server/channel.
  • Switch scope to "This server" — results now filter. Switch to "This channel" — further filter. Confirm segments are disabled when no matching selection exists.
  • Try "foo bar" (phrase), connect* (prefix), term1 AND term2 NOT term3 (boolean), sender:alice (column-scoped) — all work as documented.
  • Click a result; the sidebar selection jumps to the right channel and the sheet dismisses. For a server-console hit, the server row is selected.
  • Search for content in a previously-closed PM — confirm it doesn't appear (was cleared from the index when the PM was closed in Add SQLite FTS5 scrollback index (no UI yet) #29's closePrivateMessage).
  • Search for content in a channel from a different network — confirm it appears under "All servers" but not "This server".

Risk / rollback

• View-layer-only change. No model or persistence schema changes; index from Add SQLite FTS5 scrollback index (no UI yet) #29 is unchanged.
• Behavioral change: previous Find scanned only the in-memory channel.messages array (typically the last 500 lines after rehydrate). New search hits the full persisted index, so users will see more results — by design, but worth flagging for muscle-memory.
• Known scope-out: clicking a hit jumps to the channel but does not scroll the message buffer to the matched line. The hit row already shows timestamp + content, so the user has the context — adding scroll-to-message requires a programmatic API on MessageBufferView (NSTextView-backed) and is a separate PR.
• Revert: this single commit. The FTS5 index keeps being populated either way.