refactor(kon): clean up storage and cloud-providers — remove unused deps, add safety docs

- Remove unused chrono and uuid dependencies from kon-storage
- Add schema versioning TODO to run_migrations() for pre-release safety
- Add doc comment and usage example to log_error() (uncalled public API)
- Add TODO to consolidate app_data_dir() with model_manager::dirs_path()
- Add safety comments and #[allow(deprecated)] to keystore set_var usage
- Add TODO for keyring crate migration in cloud-providers keystore
- Apply cargo fmt

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/storage/src/file_storage.rs

@@ -3,14 +3,16 @@ use std::path::PathBuf;
 /// Resolve the app data directory.
 /// Windows: %LOCALAPPDATA%/kon
 /// Unix: ~/.kon
+///
+/// TODO: Consolidate with `crates/transcription/src/model_manager.rs::dirs_path()`
+/// into a shared helper in `crates/core/` to avoid duplicating platform-specific
+/// path logic across crates.
 pub fn app_data_dir() -> PathBuf {
     if cfg!(target_os = "windows") {
-        let local_app_data =
-            std::env::var("LOCALAPPDATA").unwrap_or_else(|_| ".".to_string());
+        let local_app_data = std::env::var("LOCALAPPDATA").unwrap_or_else(|_| ".".to_string());
         PathBuf::from(local_app_data).join("kon")
     } else {
-        let home =
-            std::env::var("HOME").unwrap_or_else(|_| "/tmp".to_string());
+        let home = std::env::var("HOME").unwrap_or_else(|_| "/tmp".to_string());
         PathBuf::from(home).join(".kon")
     }
 }