Fix the ktlint and compat-test failures, and start using the Rust gate

Two more from the step-2 removals:

**Two unused Kotlin imports** — `FilterChip` (the Note/List switch) and
`Icons.Filled.Create` (the "switch to a note" icon), both orphaned when their
callers went. ktlint treats them as errors.

**`server_info_tolerates_unknown_and_absent_fields`** pinned
`sync_protocol_version: 1` as a literal, so bumping the protocol to v2 made it
fail for a reason that has nothing to do with what it tests. It is about unknown
FIELDS; the versions now come from `CLIENT_PROTOCOL_VERSION`, like every other
test in that file already did.

The bigger fix is the habit. `ci-requirements.md` has documented since
2026-08-18 that the operator authorised running fmt/clippy/test against the CI
image locally, and I had not been doing it. All three now pass here — 116 tests,
clippy clean, fmt clean — and every Rust failure in this milestone so far would
have been caught by them in under a minute instead of by CI, several commits
downstream. Noted in ci-requirements so the next session doesn't relearn it: a
removal is exactly the change that looks too safe to check.

core/src/sync/compat.rs

@@ -344,13 +344,17 @@ mod tests {
     fn server_info_tolerates_unknown_and_absent_fields() {
         // Forward compatibility: a NEWER server sending fields we've never heard of
         // must not break the handshake.
-        let info: ServerInfo = serde_json::from_str(
-            r#"{"site_name":"S","sync_protocol_version":1,
-                "min_client_protocol_version":1,
+        // Versions come from the constants, not literals: this test is about unknown
+        // FIELDS, and pinning the numbers made it fail the moment the protocol moved
+        // to v2 — for a reason that has nothing to do with what it checks.
+        let body = format!(
+            r#"{{"site_name":"S","sync_protocol_version":{v},
+                "min_client_protocol_version":{v},
                 "sync_features":["notes","labels","attachments","tombstones","revisions"],
-                "some_future_field":{"nested":true}}"#,
-        )
-        .expect("unknown fields are ignored");
+                "some_future_field":{{"nested":true}}}}"#,
+            v = CLIENT_PROTOCOL_VERSION,
+        );
+        let info: ServerInfo = serde_json::from_str(&body).expect("unknown fields are ignored");
         assert_eq!(evaluate(&info), Compatibility::Ok);
     }