Task #501: Default new notes to yellow

The notes composer now starts with the yellow swatch pre-selected
instead of the neutral "default" color, so a freshly created note
becomes a yellow sticky by default. The user can still pick any
other color from the swatch picker before saving, and that choice
is honored. After a successful save (or send), the composer resets
back to yellow rather than "default", so the next note in the same
session is also yellow unless the user changes it.

Changes:
- artifacts/tx-os/src/pages/notes.tsx
  - Composer initial color state: useState("default") -> useState("yellow")
  - Composer reset() helper: setColor("default") -> setColor("yellow")

Out of scope (per task spec):
- No backend changes. The API's data.color = "default" fallback for
  notes created via other paths (shared-folder stamping, incoming-
  note flows) is intentionally untouched.
- No changes to existing notes already saved in the database.
- No new "default color" user setting.
- NOTE_COLORS palette and color-picker UI unchanged.

Verification:
- `pnpm exec tsc --noEmit` passes clean.
- "yellow" is a valid id in NOTE_COLORS (src/lib/notes-api.ts:801),
  so colorBg/colorAccent/colorMeta resolve correctly and the swatch
  shows pre-selected.
- Color-picker onChange path unchanged, so user-picked colors still
  override the new yellow default before save.

artifacts/tx-os/src/pages/notes.tsx

@@ -2865,7 +2865,7 @@ function Composer({
   const { t } = useTranslation();
   const [open, setOpen] = useState(false);
   const [content, setContent] = useState("");
-  const [color, setColor] = useState("default");
+  const [color, setColor] = useState("yellow");
   const [labelIds, setLabelIds] = useState<number[]>([]);
   const [kind, setKind] = useState<NoteKind>("text");
   const [items, setItems] = useState<ChecklistItem[]>([]);
@@ -2917,7 +2917,7 @@ function Composer({
 
   const reset = () => {
     setContent("");
-    setColor("default");
+    setColor("yellow");
     setLabelIds([]);
     setKind("text");
     setItems([]);