feat(app): add composer workspace footer

[Brendonovich]

Summary

• Add a darker, bordered workspace footer directly beneath the session composer, keeping the composer's own rounded shape.
• Show the current workspace and optional Git branch with compact 12px text inside a 36px-high bar. Open the existing session move menu from the workspace label to select or create a worktree.
• Share the worktree submenu across the footer, session details, and new-session selectors. Cap it at 320px, add search for 10+ worktrees, keep the search field fixed while scrolling, and support keyboard navigation and empty results.

Before / After

Same session fixture, dark theme, and 1440 × 1000 viewport.

Before	After

Story route: /iframe.html?id=app-current-session-surface--implement-and-verify-dark&viewMode=story.

Validation

• bun run check passed on the latest v2 base.
• Production-build new-session-summary.spec.ts workspace-selection scenarios passed in LTR and RTL (2 tests).
• Production cold-session benchmark passed during development; before/after measurements were recorded.
• Manually verified the live footer menu with 87 worktrees: 320px height cap, pinned search, filtering, empty results, and arrow-key navigation.
• Verified light/dark narrow layouts and RTL; footer height remains 36px with the smaller contents.