Action registry with auto-footer

[meszmate]

Summary

Centralizes app actions in a single registry (`ActionRegistry`) that becomes the single source of truth for keybindings, the command palette, help screen, and a new auto-rendered footer. Inspired by Textual's action system.

How it works

Each `Action` couples a stable string id (`"app.quit"`, `"file.save"`) to a label, description, category, optional `KeyEvent` binding, and optional handler callback. The user's `update` looks up the matching action by id and dispatches:

```zig
if (registry.matchKey(key_event)) |a| {
return self.dispatch(a.id);
}
```

The same registry can be filtered (`registry.filter(query)`) to feed a `CommandPalette`, and a `Footer` reads it to render a key hint bar that stays in sync as actions are enabled/disabled or rebound.

API surface

• `Action` — id, label, description, category, binding, enabled, show_in_footer, handler
• `ActionRegistry` — register, rebind, setEnabled, addAlias, get, matchKey, invoke, filter, formatKey
• `Footer` — auto-rendered single-line bar of footer-flagged bindings
• Aliases let multiple keys map to one action (eg. `f5` + `t` both toggle a view)

Why now

The codebase already has a `KeyMap` / `KeyBinding` pair and a `CommandPalette`. `ActionRegistry` unifies them: register once, the palette and footer pick the same data up automatically.

Test plan

• `zig build test` — added 8 tests covering register/duplicate/lookup, key matching, alias matching, fuzzy filter ranking, footer flag respected, and key formatting