Capture UX: "+ New" header button + type-to-compose (two-mode board keys)

Live-pass feedback: the wide "Take a note…" bar showed through behind the
compose modal and felt redundant. Per operator choice, remove the bar and
make capture header-button + keyboard driven.

- Removed the board's inline "Take a note…" trigger bar.
- AppShell gains a "+ New" header button (next to search) — navigates to the
  board if needed, then opens the compose modal. The `c` shortcut now does
  the same (unified with newNote()).
- Type-to-compose: on the board with no card focused, any other single
  printable key opens a new note SEEDED with that key (AppShell onKeydown
  fall-through, after the reserved / c ? g shortcuts). Seed travels via
  ui.composeSeed → NoteEditor's new `initialBody` prop; caret placed at end.
- Two-mode board keyboard (BoardView): RESTING = arrows enter browse, letters
  type-to-compose; BROWSING (a card focused) = j/k move, e/x/# act, Enter
  opens, Esc exits to resting. ui.boardCardFocused tells the global handler
  to stand down while browsing so it doesn't swallow card keys.
- Updated the shortcuts help + the empty-state copy ("Hit + New — or just
  start typing").

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

@@ -15,11 +15,17 @@ interface Toast {
 // 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.
+  // Bumped to ask the board to open the compose modal; composeSeed is the initial
+  // body (a single char for type-to-compose, or "" for the + button / `c`).
   const composeTick = ref(0);
-  function requestCompose() {
+  const composeSeed = ref("");
+  function requestCompose(seed = "") {
+    composeSeed.value = seed;
     composeTick.value++;
   }
+  // The board publishes whether a card is keyboard-focused (browse mode), so the
+  // global handler knows NOT to type-to-compose while browsing cards.
+  const boardCardFocused = ref(false);
 
   // Transient undo toast (e.g. after trash/archive).
   const toast = ref<Toast | null>(null);
@@ -47,5 +53,14 @@ export const useUiStore = defineStore("ui", () => {
     run?.();
   }
 
-  return { composeTick, requestCompose, toast, showToast, dismissToast, runToastAction };
+  return {
+    composeTick,
+    composeSeed,
+    requestCompose,
+    boardCardFocused,
+    toast,
+    showToast,
+    dismissToast,
+    runToastAction,
+  };
 });