Show thread numbers in sidebar

[ryanbbrown]

Summary

• add an app-wide, toggleable Show thread numbers setting under Settings → General
• keep the toggle off by default and persist its value across clients and restarts
• show subtle fixed-width 1–9 prefixes beside the first nine built-in sidebar threads when enabled
• derive each number from the rendered thread order so it matches the existing direct navigation commands
• expose the same toggle through bb settings general showSidebarThreadNumbers <true|false>

User impact

Thread numbers remain hidden by default. Users can turn on Settings → General → Show thread numbers to see which number opens each of the first nine visible threads. Turning the setting off hides the numbers without changing the navigation shortcuts.

Testing

• pnpm exec turbo run test --filter=@bb/app -- --run src/components/sidebar/sidebarThreadShortcuts.test.ts src/components/sidebar/ThreadRow.test.tsx
• pnpm exec turbo run test --filter=@bb/cli -- --run src/__tests__/command-output/settings.test.ts
• pnpm exec turbo run test --filter=@bb/server -- --run test/system/general-settings.test.ts
• pnpm exec turbo run test --filter=@bb/db -- --run test/migration-journal.test.ts test/migration-folder.test.ts
• pnpm exec turbo run typecheck --filter=@bb/app --filter=@bb/cli --filter=@bb/server --filter=@bb/db --filter=@bb/domain --filter=@bb/server-contract --filter=@bb/sdk --filter=@bb/plugin-sdk --filter=@bb/templates
• pnpm exec turbo run lint --filter=@bb/app --filter=@bb/cli --filter=@bb/server --filter=@bb/db --filter=@bb/domain
• git diff --check