M6: label management — usage counts + merge

Extends the existing label manager (create/rename/color/delete) with the two missing maintenance tools (task 1904), so the label list stays clean — which matters more now that #tags mint labels automatically.

Backend: GET /api/labels returns a per-label note count (one grouped query); new POST /api/labels/<id>/merge moves the source label's notes onto a target and deletes the source (repoint via delete+reinsert to avoid mutating the composite PK; preserves via_tag; dedupes notes already on the target). Body #tags are NOT rewritten, so a tag-sourced label re-mints on next edit if its #tag text remains — a documented nuance.

Frontend: LabelsModal shows each label's note count and a 'merge into…' picker; also restores the previously-missing close (x) and merge icons.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/components/Icon.vue

@@ -20,6 +20,8 @@ const paths: Record<string, string> = {
   bell: '<path d="M10.268 21a2 2 0 0 0 3.464 0"/><path d="M3.262 15.326A1 1 0 0 0 4 17h16a1 1 0 0 0 .74-1.673C19.41 13.956 18 12.499 18 8A6 6 0 0 0 6 8c0 4.499-1.411 5.956-2.738 7.326"/>',
   grip: '<circle cx="9" cy="5" r="1" fill="currentColor"/><circle cx="9" cy="12" r="1" fill="currentColor"/><circle cx="9" cy="19" r="1" fill="currentColor"/><circle cx="15" cy="5" r="1" fill="currentColor"/><circle cx="15" cy="12" r="1" fill="currentColor"/><circle cx="15" cy="19" r="1" fill="currentColor"/>',
   calendar: '<path d="M8 2v4"/><path d="M16 2v4"/><rect width="18" height="18" x="3" y="4" rx="2"/><path d="M3 10h18"/>',
+  close: '<path d="M18 6 6 18"/><path d="m6 6 12 12"/>',
+  merge: '<circle cx="18" cy="18" r="3"/><circle cx="6" cy="6" r="3"/><path d="M6 21V9a9 9 0 0 0 9 9"/>',
 };
 </script>