Add configurable thread rename shortcut

[ryanbbrown]

Closes #1157.

Summary

• expose Rename thread in Settings → Keyboard without assigning a default shortcut
• support assigning, clearing, and resetting keyboard commands that have no default shortcut
• route the command to the focused thread and reuse the existing rename dialog
• target only the focused pane when multiple threads are visible

User impact

Users can choose a keyboard shortcut for renaming the focused thread. The existing thread-title persistence behavior remains unchanged.

Testing

• pnpm exec turbo run test --filter=@bb/app -- --run src/components/settings/KeyboardSettingsSection.test.tsx src/lib/keyboard-shortcut-settings.test.ts src/views/thread-detail/ThreadRenameCommandHandler.test.tsx
• pnpm exec turbo run test --filter=@bb/server -- --run test/system/app-keybindings.test.ts
• pnpm exec turbo run typecheck --filter=@bb/app --filter=@bb/server --filter=@bb/domain --filter=@bb/server-contract --filter=@bb/plugin-sdk --filter=@bb/templates
• manual QA in the isolated development app by assigning the shortcut and renaming a thread
• git diff --check