TUI: render staleness banner, implement rescan/preview/apply handlers

[codex-devlab]

Severity: Critical (TUI advertised but largely non-functional)
Source: PR #16 review (C5, M6, M7)

rune/review/tui.py has 14 keybinds declared but most are advertised in BINDINGS without backing action_* methods. Pressing r (rescan), p (preview), a (apply), d (dismiss), o (open_editor), / (filter), ? (help), gg/G (goto) all raise in Textual.

The stale_banner_visible flag is set by _check_staleness but never rendered — compose() doesn't yield a banner widget. The "≤1s mid-session staleness banner" promise (commit 4871683) is satisfied only for tests inspecting the bool, not for users.

_label shows "[ ] finding N" with no path or kind — users can't tell what they're selecting. _detail widget is yielded but never updated.

Fix scope (likely splits into sub-tasks)

• Render banner widget bound to a reactive flag.
• Implement action_rescan (re-load chunks, clear selected, reset _initial_mtimes).
• Implement action_preview (populate _detail with chunk text + line range).
• Implement action_apply (route through applier.apply_operations with confirmation modal).
• Implement action_dismiss, action_filter, action_goto_first, action_goto_last, action_help, action_open_editor.
• Update _label to show f"{mark} {f['kind']} {Path(f['path']).name}:{f.get('start_line')}".
• Cancel set_interval on on_unmount.
• Add TUI tests asserting banner widget visibility (not just bool) and that each binding has a working handler.