feat(browser): add find in page

[smsunarto]

Summary

Add Find in Page to embedded desktop browser tabs.

• Open with ⌘F.
• Search as the user types.
• Show the current and total match count.
• Navigate with buttons, Enter, or Shift+Enter.
• Close with Escape or the close button.

Design rationale

• Use Chromium's native Find in Page engine through Electron's webContents.findInPage.
• Do not reimplement text matching, highlighting, or match navigation in BB.
• Render only the Find bar in BB because WebContentsView does not include browser chrome.
• Use BB design-system tokens and existing browser controls for the interface.
• Route ⌘F through BB's command system so it works from BB chrome and the embedded page.
• Scope requests and results by browser tab ID.
• Keep the new desktop APIs optional so older desktop shells remain compatible.
• Validate Find requests and results at the desktop IPC boundary.
• Clear native highlights when the bar closes or the page changes.

Testing

• 61 focused app, desktop, desktop-contract, and server tests pass.
• Turbo typecheck passes: 4 tasks.
• Turbo build passes: 8 tasks.
• Verified real-time search, match counts, match navigation, and closing in the live Electron app.
• Verified two-tab isolation in both directions with deterministic pages.

AGENT GENERATED: by Claude Mythos 5