feat(scribe): snippet merge UI — multi-select merge, repeatable locations

Step 3 of the snippet-merge milestone (#231): the human surfaces for
merge + multi-location, at v1 quality.

Frontend:
- SnippetListView: a Select mode (checkbox on each card) → a sticky action
  bar → a merge modal that lets you pick which selected snippet is the
  canonical (the others fold in and go to trash). Accent border on selected
  cards, Moss action buttons (Hybrid rule).
- SnippetEditorView: the single Location fieldset becomes a repeatable
  locations list (add/remove rows), so editing a merged snippet no longer
  collapses its call sites — no data loss. Sends `locations`.
- SnippetDetailView: renders every location (Location vs Locations label).
- api/snippets.ts: SnippetLocation type, `locations` on fields/input,
  mergeSnippets().

Backend (editor enablement):
- create_snippet service + POST route accept an optional `locations` list;
  PATCH route forwards `locations` — so the editor's location list works
  uniformly on create and edit. Single repo/path/symbol remain the
  one-location shorthand (MCP create contract unchanged).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pa2EsuB54BuWQ8GfJq9c7t

src/scribe/services/snippets.py

@@ -264,17 +264,20 @@ async def create_snippet(
     repo: str = "",
     path: str = "",
     symbol: str = "",
+    locations: list[dict] | None = None,
     tags: list[str] | None = None,
     project_id: int | None = None,
 ):
     """Create a snippet note (embedded on create for immediate recall). Returns
-    the created Note."""
+    the created Note. Pass ``locations`` for the multi-location case; the single
+    ``repo``/``path``/``symbol`` are the one-location shorthand."""
     note = await notes_svc.create_note(
         user_id,
         title=compose_title(name, when_to_use),
         body=compose_body(
             code=code, language=language, signature=signature,
             when_to_use=when_to_use, repo=repo, path=path, symbol=symbol,
+            locations=locations,
         ),
         note_type=SNIPPET_NOTE_TYPE,
         tags=compose_tags(language, tags),