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

tests/test_labels.py

@@ -21,3 +21,12 @@ async def test_set_note_labels_requires_auth(app):
         json={"label_ids": []},
     )
     assert resp.status_code == 401
+
+
+async def test_merge_label_requires_auth(app):
+    client = app.test_client()
+    resp = await client.post(
+        "/api/labels/00000000-0000-0000-0000-000000000000/merge",
+        json={"into": "00000000-0000-0000-0000-000000000001"},
+    )
+    assert resp.status_code == 401