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>

android/ffi/src/models.rs

@@ -334,6 +334,9 @@ pub struct Label {
     /// How many notes carry it. Only populated in listings — `None` elsewhere,
     /// matching the REST single-label responses.
     pub count: Option<i64>,
+    /// The newest edit among the untrashed notes carrying it, as the store writes
+    /// timestamps; None when nothing does. Ranks the drawer's recent tags (#5931).
+    pub last_used: Option<String>,
 }
 
 impl From<core_models::Label> for Label {
@@ -343,12 +346,14 @@ impl From<core_models::Label> for Label {
             name,
             color,
             count,
+            last_used,
         } = value;
         Label {
             id,
             name,
             color,
             count,
+            last_used,
         }
     }
 }