agent: lumotia — pin rust toolchain + workspace clippy/fmt sweep

rust-toolchain.toml pins to stable 1.94.1 so contributors and CI runners
share the exact rustc / rustfmt / clippy versions. Without the pin, every
machine surfaces a different lint set depending on its local install — six
pre-existing lints showed up on 1.94.1 that 1.93-era HANDOVER reported clean.

Clippy fixes (all pre-existing, not introduced by feature work):

- crates/storage/src/database.rs: std::iter::repeat().take() -> repeat_n()
- crates/llm/src/lib.rs (docs): "+ frontends" was parsed as a markdown bullet
  continuation by rustdoc, breaking doc-lazy-continuation. Reworded to "and".
- crates/llm/src/lib.rs (loop): while-let-on-iterator -> for-loop.
- src-tauri/src/commands/security.rs: .iter().any(|a| *a == x) -> .contains(&x).
- src-tauri/src/lib.rs: io::Error::new(Other, e) -> io::Error::other(e).
- src-tauri/src/tauri_app_data_migration.rs: drop function-tail `return`s
  inside cfg blocks; each platform's block now ends with a tail expression.

cargo fmt sweep across the workspace. Mechanical layout-only changes;
no semantics affected.

Workspace gates after this commit:
- cargo fmt --check: clean
- cargo clippy --workspace --all-targets -- -D warnings: clean
- cargo test --workspace: 405/0 (will become 409/0 with Phase A.1+A.2)

crates/storage/src/migrations.rs

@@ -1237,7 +1237,9 @@ mod tests {
         // rows so the migration doesn't accidentally mark old transcripts
         // as deleted.
         let pool = fk_test_pool().await;
-        run_migrations_up_to(&pool, 15).await.expect("migrate to v15");
+        run_migrations_up_to(&pool, 15)
+            .await
+            .expect("migrate to v15");
 
         // Seed a pre-v16 row to verify backfill preserves NULL.
         sqlx::query(
@@ -1290,7 +1292,9 @@ mod tests {
         .await
         .expect("read indexes");
         assert!(
-            index_names.iter().any(|n| n == "idx_transcripts_deleted_at"),
+            index_names
+                .iter()
+                .any(|n| n == "idx_transcripts_deleted_at"),
             "expected idx_transcripts_deleted_at, got {index_names:?}",
         );
     }