Core: tag names fold every letter, as the server does (#5385)

SQLite's lower() folds ASCII only, so a device could hold "Café" and
"CAFÉ" as two tags while the server held one. derive::fold (to_lowercase)
is now the one comparison. It is registered as the deterministic SQL function
fold() on every connection (schema::migrate), and find_or_create, the rename
clash, the pull clash and the unique index all use it. derive's push_unique
used eq_ignore_ascii_case and now folds the same way.

v14 merges pairs a device already holds before rebuilding the index. The
older row survives, as in rename_label. Memberships move with via_tag kept,
affected notes go dirty, and the merged-away row is a pending delete. It is
written out rather than calling store::merge_labels so the migration doesn't
depend on store code that later versions may change.

rusqlite gains its "functions" feature (operator-approved, 2026-10-08).
grammar.json gains "#café and #CAFÉ" -> ["café"], which all three
implementations run.

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

core/src/local/store.rs

@@ -248,7 +248,7 @@ fn touch(conn: &Connection, id: &str) -> rusqlite::Result<()> {
 fn find_or_create_label(conn: &Connection, name: &str) -> rusqlite::Result<String> {
     let existing: Option<String> = conn
         .query_row(
-            "SELECT id FROM labels WHERE lower(name) = lower(?1)",
+            "SELECT id FROM labels WHERE fold(name) = fold(?1)",
             [name],
             |r| r.get(0),
         )
@@ -1039,7 +1039,7 @@ pub fn create_label(conn: &Connection, name: &str) -> rusqlite::Result<Label> {
 pub fn rename_label(conn: &Connection, id: &str, name: &str) -> rusqlite::Result<Label> {
     let clash: Option<(String, String)> = conn
         .query_row(
-            "SELECT id, created_at FROM labels WHERE lower(name) = lower(?1) AND id <> ?2",
+            "SELECT id, created_at FROM labels WHERE fold(name) = fold(?1) AND id <> ?2",
             params![name, id],
             |r| Ok((r.get(0)?, r.get(1)?)),
         )
@@ -1580,6 +1580,18 @@ mod tests {
         assert!(edited.labels.is_empty());
     }
 
+    /// Every letter folds, accented ones included, as on the server (#5385). With
+    /// SQLite's lower() the second note minted a "CAFÉ" tag of its own.
+    #[test]
+    fn a_tag_spelled_with_different_accented_capitals_is_the_same_tag() {
+        let conn = db();
+        let first = note(&conn, "coffee at #Café");
+        let second = note(&conn, "coffee at #CAFÉ");
+        assert_eq!(second.labels.len(), 1);
+        assert_eq!(second.labels[0].id, first.labels[0].id);
+        assert_eq!(list_labels(&conn).expect("labels").len(), 1);
+    }
+
     // ---- writes ----------------------------------------------------------------
 
     #[test]