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)

src-tauri/src/commands/security.rs

@@ -21,15 +21,12 @@ pub fn ensure_main_window_label(label: &str) -> Result<(), String> {
 /// `&[&'static str]` and should mirror an entry in
 /// `src-tauri/capabilities/*.json` — keeping the IPC trust boundary and the
 /// permission grant in lock-step.
-pub fn ensure_window_in_set(
-    window: &tauri::WebviewWindow,
-    allowed: &[&str],
-) -> Result<(), String> {
+pub fn ensure_window_in_set(window: &tauri::WebviewWindow, allowed: &[&str]) -> Result<(), String> {
     ensure_window_in_set_label(window.label(), allowed)
 }
 
 pub fn ensure_window_in_set_label(label: &str, allowed: &[&str]) -> Result<(), String> {
-    if allowed.iter().any(|a| *a == label) {
+    if allowed.contains(&label) {
         Ok(())
     } else {
         Err(format!(