Keyboard shortcuts system with Cmd-hold hints (PaperShelf pattern)

[dakl]

Add comprehensive, discoverable, configurable keyboard shortcuts. When Cmd is held, show shortcut hints overlaid on UI elements (PaperShelf pattern, ~200ms delay, fade in). Shortcuts settable in Settings with conflict detection. Defaults: Cmd+N new session, Cmd+W close, Cmd+Up/Down navigate sessions, Cmd+1-9 switch by index, Cmd+Enter send. Port the Zustand shortcut store from PaperShelf (defaults, user overrides, conflict detection, Cmd-hold hint visibility).