fix: resolve UI hangs and contrast issues, improve concurrency

- Prevent UI hang by processing aria2 unpause actions asynchronously
- Bypass internal aria2 limits to honor Firelink's max concurrent setting
- Improve visibility of the 'Recheck engines' button in Settings
- Refactor Speed Limiter checkbox into a toggle and fix text selection
- Update Add window font and placeholder to match modern design standards

src-tauri/src/lib.rs

@@ -2396,30 +2396,45 @@ async fn resume_download(
     let status = aria2_download_status(state.aria2_port, &state.aria2_secret, &gid).await?;
     match status.as_str() {
         "paused" => {
-            let acquired = state.queue_manager.ensure_aria2_permit(&id).await;
-            let result = match rpc_call(
-                state.aria2_port,
-                &state.aria2_secret,
-                "aria2.unpause",
-                serde_json::json!([gid]),
-            )
-            .await
-            {
-                Ok(result) => result,
-                Err(error) => {
-                    if acquired {
-                        state.queue_manager.release_permit(&id).await;
+            use tauri::Emitter;
+            let _ = state.emit(
+                "download-state",
+                crate::ipc::DownloadStateEvent::new(&id, crate::ipc::DownloadStatus::Queued),
+            );
+            
+            let app_state = state.inner().clone();
+            let id_clone = id.clone();
+            let gid_clone = gid.clone();
+            
+            tauri::async_runtime::spawn(async move {
+                let acquired = app_state.queue_manager.ensure_aria2_permit(&id_clone).await;
+                let result = match rpc_call(
+                    app_state.aria2_port,
+                    &app_state.aria2_secret,
+                    "aria2.unpause",
+                    serde_json::json!([gid_clone]),
+                )
+                .await
+                {
+                    Ok(result) => result,
+                    Err(error) => {
+                        if acquired {
+                            app_state.queue_manager.release_permit(&id_clone).await;
+                        }
+                        log::error!("failed to resume aria2 gid {}: {}", gid_clone, error);
+                        return;
                     }
-                    return Err(format!("failed to resume aria2 gid {gid}: {error}"));
+                };
+                if let Err(error) = ensure_aria2_gid_result("unpause", &gid_clone, &result) {
+                    if acquired {
+                        app_state.queue_manager.release_permit(&id_clone).await;
+                    }
+                    log::error!("failed to resume aria2 gid {}: {}", gid_clone, error);
+                    return;
                 }
-            };
-            if let Err(error) = ensure_aria2_gid_result("unpause", &gid, &result) {
-                if acquired {
-                    state.queue_manager.release_permit(&id).await;
-                }
-                return Err(error);
-            }
-            log::info!("aria2 resume [{}]: unpaused gid {}", id, gid);
+                log::info!("aria2 resume [{}]: unpaused gid {}", id_clone, gid_clone);
+            });
+            return Ok(true);
         }
         "active" | "waiting" => {
             state.queue_manager.ensure_aria2_permit(&id).await;
@@ -3960,6 +3975,7 @@ pub fn run() {
                         .arg("--summary-interval=1")
                         .arg("--console-log-level=warn")
                         .arg("--download-result=hide")
+                        .arg("--max-concurrent-downloads=9999")
                         .arg("--check-certificate=true");
 
             if let Some(global_speed_limit) = normalize_speed_limit_for_aria2(&global_speed_limit) {

src/components/AddDownloadsModal.tsx

@@ -734,8 +734,8 @@ export const AddDownloadsModal = () => {
                   </div>
                 </div>
                 <textarea
-                  className="add-download-control add-download-links-input w-full h-32 p-3 text-[13px] resize-none font-mono"
-                  placeholder="Paste HTTP, HTTPS, FTP, or SFTP URLs here..."
+                  className="add-download-control add-download-links-input w-full h-32 p-3 text-[13px] resize-none"
+                  placeholder={"Paste HTTP, HTTPS, FTP, or SFTP URLs here...\n\nFor media downloads, paste links from Youtube, X, TikTok, Instagram, Reddit, etc."}
                   value={urls}
                   onChange={(e) => setUrls(e.target.value)}
                 />

src/components/SettingsView.tsx

@@ -971,7 +971,7 @@ runEngineChecks(false);
 type="button"
 onClick={() => runEngineChecks(true)}
 disabled={isRecheckingEngines}
-className="flex items-center gap-1.5 px-3 py-1.5 rounded-md bg-bg-modal hover:bg-item-hover disabled:opacity-60 text-[12px] text-text-primary"
+className="app-button px-3 py-1.5 text-[12px] flex items-center gap-1.5 disabled:opacity-60"
 >
 <RefreshCw size={13} className={isRecheckingEngines ? 'animate-spin' : ''} />
 {isRecheckingEngines ? 'Checking…' : 'Recheck engines'}

src/components/SpeedLimiterView.tsx

@@ -58,10 +58,19 @@ export default function SpeedLimiterView() {
       <WindowDragRegion />
 
       <div className="flex items-center gap-3 border-b border-border-color px-6 pb-4">
-        <label className="flex items-center gap-3 text-[17px] font-semibold tracking-tight text-text-primary">
-          <input type="checkbox" checked={enabled} onChange={event => setEnabled(event.target.checked)} className="h-4 w-4 accent-accent" />
+        <div className="flex items-center gap-3 text-[17px] font-semibold tracking-tight text-text-primary select-none">
+          <button
+            onClick={() => setEnabled(!enabled)}
+            className={`relative inline-flex h-5 w-9 cursor-pointer items-center rounded-full transition-colors duration-200 ease-in-out focus:outline-none ${enabled ? 'bg-accent' : 'bg-item-hover'}`}
+            aria-checked={enabled}
+            role="switch"
+          >
+            <span
+              className={`inline-block h-4 w-4 transform rounded-full bg-white transition duration-200 ease-in-out ${enabled ? 'translate-x-4' : 'translate-x-1'}`}
+            />
+          </button>
           Speed Limiter
-        </label>
+        </div>
         <span className={`rounded-full px-2.5 py-1 text-[11px] font-semibold ${
           enabled ? 'bg-accent/15 text-accent' : 'bg-item-hover text-text-muted'
         }`}>