fix: resolve download state, toast UI, and checksum cleanup loopholes

- Prevent backward state transitions for downloads in frontend store.
- Add Downloading event emission upon aria2 unpause.
- Fix toast container visual bleed on exit by adding overflow-hidden.
- Add retry backoff for deleted files to prevent Windows file lock errors.
- Lowercase checksum hash types before passing to aria2.
- Clean up lingering files upon checksum verification failure.
- Convert Tauri AppHandle arguments to support generic runtimes.

src-tauri/src/settings.rs

@@ -6,7 +6,7 @@ use serde_json::{Map, Value};
 use std::collections::HashMap;
 use tauri::{AppHandle, Manager};
 
-pub fn load_settings(app_handle: &AppHandle) -> Result<PersistedSettings, String> {
+pub fn load_settings<R: tauri::Runtime>(app_handle: &AppHandle<R>) -> Result<PersistedSettings, String> {
     let database = app_handle.state::<crate::db::DbState>();
     let connection = database.lock()?;
     let stored = crate::db::load_settings(&connection)?