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/i18n/locales/en/card.json

@@ -8,6 +8,7 @@
   "unhide": "Unhide",
   "hide": "Hide",
   "onlyThisChannel": "Only this channel",
+  "resetState": "Reset — clear watch progress/status",
   "thisChannel": "This channel",
   "continueTitle": "Continue where you left off",
   "continue": "Continue",

frontend/src/i18n/locales/en/channels.json

@@ -13,6 +13,7 @@
   },
   "tags": {
     "yourTags": "Your tags",
+    "manage": "Manage tags",
     "yourTagsHint": "Your personal labels. Attach them to channels below, then filter the feed by tag from the sidebar. (Separate from the automatic language/topic tags.)",
     "newTag": "new tag",
     "createTag": "Create tag"
@@ -56,6 +57,7 @@
     "subs": "{{formatted}} subs",
     "openOnYouTube": "Open on YouTube",
     "editTags": "Edit tags",
+    "filterFeedByTag": "Filter the feed by “{{name}}”",
     "priorityHint": "Your ranking for this channel. Sort the feed by “Channel priority” to bring higher-priority channels to the top.",
     "raisePriority": "Raise priority",
     "lowerPriority": "Lower priority",
@@ -94,6 +96,7 @@
     "fullHistoryFailed": "Couldn't request full history",
     "needYouTube": "Connect your YouTube account to do this.",
     "connect": "Connect",
+    "filteredByTag": "Feed filtered by tag: {{name}}",
     "resetDone": "Channel reset — re-fetching now",
     "resetFailed": "Couldn't reset this channel"
   },

frontend/src/i18n/locales/en/sidebar.json

@@ -26,6 +26,7 @@
   "clearDates": "clear dates",
   "reshuffle": "Reshuffle",
   "noMatchingTags": "No matching tags here",
+  "manageTags": "Manage",
   "shareView": "Copy a link to this view",
   "shareCopied": "View link copied to clipboard",
   "shareFailed": "Couldn't copy the link",
@@ -35,7 +36,8 @@
     "date": "Upload date",
     "content": "Content type",
     "language": "Language",
-    "topic": "Topic"
+    "topic": "Topic",
+    "tags": "Your tags"
   },
   "show": {
     "unwatched": "Unwatched",

frontend/src/i18n/locales/en/tagManager.json

@@ -0,0 +1,12 @@
+{
+  "title": "Manage tags",
+  "channels": "{{count}} ch.",
+  "onChannels": "Tagged channels",
+  "delete": "Delete",
+  "deleted": "Tag deleted",
+  "deleteTitle": "Delete tag",
+  "confirmDelete": "Delete the tag “{{name}}”? It will be removed from every channel it's on.",
+  "empty": "No tags yet — add one below.",
+  "newPlaceholder": "New tag name…",
+  "add": "Add"
+}