Capture: Enter-to-start + on-board notice; dismiss discards a new note

Live-pass feedback: make the "waiting for input" state visible and starting
a note more deliberate, and let dismiss cancel an accidental note.

- New notes are now a confirm-to-keep dialog: Esc OR click-away DISCARD a
  brand-new, not-yet-persisted note (so an accidental keystroke / type-to-
  compose never litters); Ctrl/Cmd+Enter, the footer button, or Shift+Enter
  commit it. A compose already persisted by a rich action, and any existing
  note, still close-and-save on dismiss. The compose footer button is now a
  filled "Add note" so the save path is unmistakable.
- Enter (board, nothing focused) starts a new note; a subtle dashed on-board
  notice — "Press Enter or start typing to add a note" — makes capture
  discoverable instead of silent (click it to compose too). Type-to-compose
  stays as the fast path.
- Empty-state copy + shortcuts help updated (Enter/c for a new note).

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

frontend/src/components/AppShell.vue

@@ -42,7 +42,7 @@ let searchTimer: ReturnType<typeof setTimeout> | undefined;
 
 const shortcuts = [
   { label: "Command palette", keys: ["⌘/Ctrl", "K"] },
-  { label: "New note (or just start typing)", keys: ["c"] },
+  { label: "New note (or just start typing)", keys: ["Enter", "c"] },
   { label: "Search", keys: ["/"] },
   { label: "Go to Board", keys: ["g", "b"] },
   { label: "Go to Graph", keys: ["g", "g"] },