feat(app): add find in pane across panes

[boudra]

Summary

• Shipped shared find-in-pane scaffolding: pane-scoped registry, keyboard routing, shared FindBar, lifecycle cleanup, pending/no-match/match states, and focused-pane ownership.
• Added per-pane search models and adapters for terminal, file, agent stream, and browser panes, including highlights, counters, next/prev navigation, empty query cleanup, and targeted regression coverage.
• Fixed Electron desktop browser find to route through the main-process owner WebContents bridge, with bridge unit coverage and diagnostic QA harness updates.

Manual test plan

• Terminal pane Cmd+F: open find, type a matching query, use Enter, Shift+Enter, next/prev buttons, Esc, close, empty query, and no-match.
• File pane Cmd+F: open find on a text/code file, verify highlights, counter, wrap navigation, buttons, Esc, close, empty query, and no-match.
• Agent stream Cmd+F: search loaded chat/live stream text, verify navigation/highlights, pane switching cleanup, buttons, Esc, close, empty query, and no-match.
• Web browser pane Cmd+F (browser fallback): verify the shared find UI opens without Electron webview APIs and degrades gracefully.
• Electron desktop browser pane Cmd+F: run the real Electron app, open a browser pane, search page text through the webview bridge, verify counter/navigation/cleanup. This must be real Electron, not Playwright-only evidence.

QA note

Automated Playwright Electron QA was attempted, but it produced inconsistent results because CDP/keyboard interactions did not reliably exercise Electron's find subsystem or deliver found-in-page events in the harness. The Electron bridge is verified by inspection plus bridge/unit tests; a real Electron manual smoke pass is still needed.

[boudra]

Applied the four-way find-in-pane cleanup pass in 9e478a0.

• A bridge: removed renderer webview find fallbacks; Electron find now requires the preload bridge.
• B core: tightened pane-find/registry tests around visible behavior and dropped internal exports where typecheck allowed it.
• C adapters: chat/file find now indexes/registers only highlightable/searchable content; C8 deferred as requested.
• D harness: deleted the flaky Electron E2E harness and QA doc; tightened the web spec and removed find from the dispatcher type.

Delta: 19 files changed, 221 insertions(+), 1924 deletions(-).
Verification: npm run typecheck, npm run lint, npm run format, and the targeted 14-file Vitest run are green.