Replace keyboard system with TanStack Hotkeys and jotai atoms

Remove react-hotkeys-hook, KeyboardProvider context, useRegisterShortcut
wrapper, and KeyboardHelpDialog component. Components now call TanStack
useHotkey/useHotkeySequence directly. Shared state uses jotai atoms in
lib/atoms/, shortcut metadata lives as a static const in lib/constants/.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hooks/use-register-shortcut.ts

@@ -1,14 +0,0 @@
-import { useEffect } from "react";
-import { type ShortcutDef, useKeyboard } from "@/components/keyboard-provider";
-
-export function useRegisterShortcut(id: string, def: ShortcutDef) {
-  const { registerShortcut, unregisterShortcut } = useKeyboard();
-
-  useEffect(() => {
-    registerShortcut(id, def);
-  });
-
-  useEffect(() => {
-    return () => unregisterShortcut(id);
-  }, [id, unregisterShortcut]);
-}