feat(tags): tag UX overhaul + per-card video reset

- Channel-row tags show only what's attached; a '+' opens a per-channel picker. Clicking a tag
  filters the feed by it; a 'Your tags' sidebar widget makes that filter visible/clearable.
- Tag manager dialog (add/rename/delete; delete only confirms when the tag is in use), reachable
  from the Channel manager and the feed sidebar; hovering a tag's count lists its channels, each a
  link that focuses it in the Channel manager (DataTable gains an external filter input).
- Video cards get a reset action that clears all watch state (incl. an in-progress position).
- api.req() now raises the error dialog on 5xx and 400/409/422 with the server's reason.

frontend/src/components/VideoCard.tsx

@@ -19,11 +19,13 @@ import { formatDuration, formatViews, relativeTime } from "../lib/format";
 function Actions({
   video,
   onState,
+  onResetState,
   onToggleSave,
   onChannelFilter,
 }: {
   video: Video;
   onState: (id: string, status: string) => void;
+  onResetState?: (id: string) => void;
   onToggleSave: (id: string, saved: boolean) => void;
   onChannelFilter?: (channelId: string, channelName: string) => void;
 }) {
@@ -33,6 +35,9 @@ function Actions({
     e.stopPropagation();
     onState(video.id, video.status === status ? "new" : status);
   };
+  // Pristine = never opened: default status and no resume position. The reset clears the
+  // whole state (incl. an in-progress position the un-watch toggle can't touch).
+  const resettable = video.status !== "new" || video.position_seconds > 0;
   const toggleSave = (e: React.MouseEvent) => {
     e.preventDefault();
     e.stopPropagation();
@@ -79,6 +84,19 @@ function Actions({
           <EyeOff className="w-4 h-4" />
         )}
       </button>
+      {onResetState && resettable && (
+        <button
+          onClick={(e) => {
+            e.preventDefault();
+            e.stopPropagation();
+            onResetState(video.id);
+          }}
+          title={t("card.resetState")}
+          className="p-1.5 rounded-md hover:bg-surface text-muted hover:text-fg"
+        >
+          <RotateCcw className="w-4 h-4" />
+        </button>
+      )}
       {onChannelFilter && (
         <button
           onClick={(e) => {
@@ -224,6 +242,7 @@ function VideoCard({
   video,
   view,
   onState,
+  onResetState,
   onToggleSave,
   onChannelFilter,
   onOpen,
@@ -231,6 +250,7 @@ function VideoCard({
   video: Video;
   view: "grid" | "list";
   onState: (id: string, status: string) => void;
+  onResetState?: (id: string) => void;
   onToggleSave: (id: string, saved: boolean) => void;
   onChannelFilter?: (channelId: string, channelName: string) => void;
   onOpen?: (v: Video, startAt?: number | null) => void;
@@ -294,7 +314,7 @@ function VideoCard({
           </a>
           <div className="text-xs text-muted mt-0.5">{meta}</div>
         </div>
-        <Actions video={video} onState={onState} onToggleSave={onToggleSave} onChannelFilter={onChannelFilter} />
+        <Actions video={video} onState={onState} onResetState={onResetState} onToggleSave={onToggleSave} onChannelFilter={onChannelFilter} />
       </div>
     );
   }
@@ -325,7 +345,7 @@ function VideoCard({
             {video.channel_title}
           </a>
           <div className="text-xs text-muted mt-0.5">{meta}</div>
-          <Actions video={video} onState={onState} onToggleSave={onToggleSave} onChannelFilter={onChannelFilter} />
+          <Actions video={video} onState={onState} onResetState={onResetState} onToggleSave={onToggleSave} onChannelFilter={onChannelFilter} />
         </div>
       </div>
     </div>