feat: add resumability detection and pause warnings

- Update 'fetch_metadata' to detect 'Accept-Ranges' HTTP header
- Persist 'resumable' flag in DownloadItem across backend and frontend
- Add confirmation dialog when pausing non-resumable downloads (single and bulk actions)
- Improve UI for start/pause/options buttons by adopting the application accent color on hover

src-tauri/src/ipc.rs

@@ -79,6 +79,8 @@ pub struct DownloadItem {
     pub category: DownloadCategory,
     pub date_added: String,
     #[ts(optional)]
+    pub resumable: Option<bool>,
+    #[ts(optional)]
     pub connections: Option<i32>,
     #[ts(optional)]
     pub speed_limit: Option<String>,