m4.5: lists first-class — create a checklist straight from quick-add

Checklist notes existed (M2) but could only be made by creating a text
note and toggling it in the editor — so they were undiscoverable. Now
the board's quick-add can make one in one shot.

- create endpoint accepts kind + items: POST /api/notes with
  kind:"list" and items:[...] creates a checklist note and its items
  atomically. A list note is non-empty when it has a title or ≥1 item.
- quick-add gets a checklist toggle (checkbox icon): flip it and each
  body line becomes an item on save; placeholder switches to
  "One item per line"; resets to a plain note after close.
- notes store create() accepts kind + items; parse_list_items() helper
  (trims, drops blanks) with a unit test.
- title placeholders now read "Title (optional)" in quick-add too.

Third item of M4.5. Display/editing of checklists was already built in
M2; this closes the creation gap.

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

frontend/src/stores/notes.ts

@@ -95,7 +95,13 @@ export const useNotesStore = defineStore("notes", () => {
     }
   }
 
-  async function create(input: { title: string; body: string; color: NoteColor }): Promise<void> {
+  async function create(input: {
+    title: string;
+    body: string;
+    color: NoteColor;
+    kind?: NoteKind;
+    items?: string[];
+  }): Promise<void> {
     reconcile(await api.post<Note>("/api/notes", input));
   }