keyboard: global shortcuts + ? cheat-sheet

App-shell keydown dispatcher (active only inside the authed shell):
- c        new note (jump to board / reopen composer via a ui-store signal)
- /        focus the search box
- g b/g/r  jump to Board / Graph / Reminders (two-key, 800ms window)
- ?        open a keyboard-shortcuts cheat-sheet overlay
- Esc      blur a focused field / close the cheat-sheet

Ignored while typing in an input/textarea/contenteditable (except Esc).
New stores/ui.ts carries the compose signal; QuickAdd exposes open(); the
board reopens the composer on the signal when already on the board.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/stores/ui.ts

@@ -0,0 +1,14 @@
+import { defineStore } from "pinia";
+import { ref } from "vue";
+
+// Cross-component UI signals that don't belong to any single view (e.g. a
+// global shortcut in the app shell asking the board to act).
+export const useUiStore = defineStore("ui", () => {
+  // Bumped to ask the board to open + focus its quick-add composer.
+  const composeTick = ref(0);
+  function requestCompose() {
+    composeTick.value++;
+  }
+
+  return { composeTick, requestCompose };
+});