ui: .btn:disabled styling — visual cue for disabled queue-action buttons

The .btn base class (Queue actions: Start / Wiederholen / Leeren / Merge & Split) had no :disabled rule, so when the renderer set btnRetryFailed.disabled = true (no failed downloads in queue), the button visually looked identical to its enabled state — clickable-styled but rejecting clicks. The mouse cursor stayed `pointer` even though the button did nothing.

Added .btn:disabled with the standard 0.45 opacity + cursor:not-allowed (matching .btn-pill:disabled from 4.6.21-era styling). The .btn:disabled:hover { background: inherit; } companion stops the per-variant hover rules (.btn-retry:hover, .btn-start:hover, etc.) from overriding the disabled background — the disabled state stays inert under the cursor.

Affects btnRetryFailed (most visible — toggles disabled when no items are in the error state) and is also robustness against any future code that disables btnStart / btnClear / btnMergeGroup.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/styles.css

@@ -1231,6 +1231,15 @@ select option {
     transition: all 0.2s;
 }
 
+.btn:disabled {
+    opacity: 0.45;
+    cursor: not-allowed;
+}
+
+.btn:disabled:hover {
+    background: inherit;
+}
+
 .btn:focus-visible {
     outline: none;
     box-shadow: 0 0 0 2px rgba(145, 70, 255, 0.65);