Auto-generate session name when user creates without typing one

Click "Create session" with the input empty and a name of the form
`YYMMDD-session-NNN` is generated automatically: today's date as
two-digit year/month/day, then a zero-padded counter that starts at
000 and increments past the highest existing match for the same day.

Added:
- `computeNextDefaultName(existing, now?)` pure helper exported from
  `@engine/services/sessions`.
- `SessionService.nextDefaultName(now?)` method that wraps it
  against the current repo.
- Both create call sites (CreateFirstSession empty state +
  SessionMenu's New session form) fall back to `service.nextDefaultName()`
  when the trimmed input is empty.
- 5 new unit tests covering today-only counting, max-not-count
  increment, and trimmed/wrong-shape filtering.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/sessions/CreateFirstSession.tsx

@@ -33,7 +33,9 @@ export function CreateFirstSession() {
   const handleCreate = useCallback(() => {
     setError(null);
     try {
-      const created = service.create(name);
+      const trimmed = name.trim();
+      const effective = trimmed.length === 0 ? service.nextDefaultName() : name;
+      const created = service.create(effective);
       navigateTo({ sessionId: created.id, mode: "review" });
     } catch (err) {
       setError(err instanceof Error ? err.message : String(err));

src/app/sessions/SessionMenu.tsx

@@ -104,7 +104,9 @@ export function SessionMenu({ onExportZip, onImportZip, onOpenSamples }: Session
   const handleCreate = useCallback(() => {
     setError(null);
     try {
-      const created = service.create(newName);
+      const trimmed = newName.trim();
+      const effective = trimmed.length === 0 ? service.nextDefaultName() : newName;
+      const created = service.create(effective);
       setNewName("");
       setCreating(false);
       setOpen(false);