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/transcription/src/whisper_rs_backend.rs

@@ -92,9 +92,7 @@ impl WhisperRsBackend {
         );
 
         let mut state = self.ctx.create_state().map_err(|e| {
-            Error::TranscriptionFailed(
-                WhisperBackendError::State(e.to_string()).to_string(),
-            )
+            Error::TranscriptionFailed(WhisperBackendError::State(e.to_string()).to_string())
         })?;
 
         let mut params = FullParams::new(SamplingStrategy::Greedy { best_of: 1 });
@@ -124,9 +122,7 @@ impl WhisperRsBackend {
         }
 
         state.full(params, samples).map_err(|e| {
-            Error::TranscriptionFailed(
-                WhisperBackendError::Transcribe(e.to_string()).to_string(),
-            )
+            Error::TranscriptionFailed(WhisperBackendError::Transcribe(e.to_string()).to_string())
         })?;
 
         let n = state.full_n_segments();