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/routes/snippets.py

@@ -73,6 +73,7 @@ async def create_snippet_route():
         repo=data.get("repo", ""),
         path=data.get("path", ""),
         symbol=data.get("symbol", ""),
+        locations=data.get("locations"),
         tags=data.get("tags"),
         project_id=project_id,
     )
@@ -109,6 +110,8 @@ async def update_snippet_route(snippet_id: int):
     # treats None as "leave unchanged"). Empty strings ARE applied — the form
     # sends the full field set, so a cleared field is an intentional clear.
     kwargs = {k: data[k] for k in _STR_FIELDS if k in data}
+    if "locations" in data:
+        kwargs["locations"] = data["locations"]
     if "tags" in data:
         kwargs["tags"] = data["tags"]
     if "project_id" in data: