Add configurable thread archive shortcut

[ryanbbrown]

Depends on #1158.

Summary

• expose Archive thread in Settings → Keyboard without assigning a default shortcut
• route the command to the focused thread and reuse the existing archive lifecycle, navigation, and toast behavior
• target only the focused pane when multiple threads are visible
• ignore archived threads instead of turning the command into an archive/unarchive toggle

User impact

Users can choose a keyboard shortcut for archiving the focused thread. Existing archive behavior remains unchanged, including child handling and pane cleanup.

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/ThreadArchiveCommandHandler.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 archiving focused threads in single-pane and split views
• git diff --check