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

frontend/src/views/SnippetDetailView.vue

@@ -21,11 +21,11 @@ const canWrite = computed(() => {
   return p === undefined || p === "owner" || p === "edit" || p === "admin";
 });
 
-const locationParts = computed(() => {
-  const s = snippet.value?.snippet;
-  if (!s) return [];
-  return [s.repo, s.path, s.symbol].filter((p) => p && p.trim());
-});
+const locations = computed(() => snippet.value?.snippet.locations ?? []);
+
+function locParts(loc: { repo: string; path: string; symbol: string }): string[] {
+  return [loc.repo, loc.path, loc.symbol].filter((p) => p && p.trim());
+}
 
 async function load() {
   loading.value = true;
@@ -92,10 +92,12 @@ async function confirmDelete() {
           <dt>Signature</dt>
           <dd><code>{{ snippet.snippet.signature }}</code></dd>
         </template>
-        <template v-if="locationParts.length">
-          <dt>Location</dt>
-          <dd class="location">
-            <code v-for="(p, i) in locationParts" :key="i">{{ p }}</code>
+        <template v-if="locations.length">
+          <dt>{{ locations.length > 1 ? "Locations" : "Location" }}</dt>
+          <dd class="location-list">
+            <div v-for="(loc, i) in locations" :key="i" class="location">
+              <code v-for="(p, j) in locParts(loc)" :key="j">{{ p }}</code>
+            </div>
           </dd>
         </template>
         <template v-if="snippet.snippet.language">
@@ -242,6 +244,11 @@ async function confirmDelete() {
   border-radius: var(--radius-sm);
   word-break: break-all;
 }
+.location-list {
+  display: flex;
+  flex-direction: column;
+  gap: 0.35rem;
+}
 .location {
   display: flex;
   flex-wrap: wrap;