Rail and drawer: only the 5 tags most recently used or viewed (#5931)

A long tag list pushed the drawer's Archive, Trash and Sync off screen on
Android, where Tags still sat above them (#5915 only reordered the web rail).
Tags now come last on Android too, and both clients list the five tags most
recently used or viewed, by name, with "All tags (N)" opening the rest in
place.

Used: the newest untrashed note carrying the tag. GET /api/labels and the
core's list_labels return it as last_used, and each client also folds in the
notes it holds, so a tag applied a moment ago counts before the next listing.
Viewed: opening the tag's lens, remembered on the device (per user on the web).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_integration.py

@@ -665,6 +665,26 @@ async def _push_label(app_client, lid: str, op: str, edited_at: str, name: str =
     return (await resp.get_json())["results"][0]
 
 
+async def test_a_tag_listing_says_when_each_was_last_used(app_client, db):
+    """The rail keeps the most recent few (#5931): a tag is as recent as the newest
+    note carrying it, and a note in the trash does not count."""
+    await _signed_in(app_client, "lastused")
+    used = await (await app_client.post("/api/labels", json={"name": "errands"})).get_json()
+    idle = await (await app_client.post("/api/labels", json={"name": "someday"})).get_json()
+    binned = await (await app_client.post("/api/labels", json={"name": "old"})).get_json()
+    note = await (await app_client.post("/api/notes", json={"body": "milk"})).get_json()
+    await app_client.put(f"/api/notes/{note['id']}/labels", json={"label_ids": [used["id"]]})
+    gone = await (await app_client.post("/api/notes", json={"body": "stale"})).get_json()
+    await app_client.put(f"/api/notes/{gone['id']}/labels", json={"label_ids": [binned["id"]]})
+    assert (await app_client.post(f"/api/notes/{gone['id']}/trash")).status_code == 200
+
+    listing = {lb["id"]: lb for lb in (await (await app_client.get("/api/labels")).get_json())["labels"]}
+    current = (await (await app_client.get(f"/api/notes/{note['id']}")).get_json())["updated_at"]
+    assert listing[used["id"]]["last_used"] == current
+    assert listing[idle["id"]]["last_used"] is None
+    assert listing[binned["id"]]["last_used"] is None
+
+
 async def test_a_tag_deleted_on_the_web_reaches_devices_as_a_tombstone(app_client, db):
     await _signed_in(app_client, "webdelete")
     tag = await (await app_client.post("/api/labels", json={"name": "grocery"})).get_json()