feat(watchlist): add watchlist toggle to library context menus

Watchlist membership was only reachable from Explore cards and the
detail screen's action row, which drops the bookmark first on narrow
screens with no fallback in the overflow menu. Add an entry to
MediaContextMenu for movies and shows whenever a connected catalog
source can hold the item, covering card long-press everywhere and the
detail screen's overflow.

External-id resolution is session-cached per item on
CatalogSourcesProvider and shared with the detail screen. A cold cache
labels the entry "Add to Watchlist" and always adds (idempotent), so a
press can never turn into a surprise removal; "Remove" is offered once
cached membership proves it. Several capable sources open the same
per-source chooser the detail screen uses.

close #1822

lib/i18n/strings_en.g.dart

@@ -3054,9 +3054,18 @@ class Translations$explore$en {
 	/// en: 'Remove from Watchlist'
 	String get removeFromWatchlist => 'Remove from Watchlist';
 
+	/// en: 'Added to watchlist'
+	String get addedToWatchlist => 'Added to watchlist';
+
+	/// en: 'Removed from watchlist'
+	String get removedFromWatchlist => 'Removed from watchlist';
+
 	/// en: 'Couldn't update watchlist'
 	String get watchlistUpdateFailed => 'Couldn\'t update watchlist';
 
+	/// en: 'Couldn't match this item to a watchlist'
+	String get watchlistNoMatch => 'Couldn\'t match this item to a watchlist';
+
 	/// en: 'Not in your library'
 	String get notInLibrary => 'Not in your library';
 
@@ -7065,7 +7074,10 @@ extension on Translations {
 			'explore.characters' => 'Characters',
 			'explore.addToWatchlist' => 'Add to Watchlist',
 			'explore.removeFromWatchlist' => 'Remove from Watchlist',
+			'explore.addedToWatchlist' => 'Added to watchlist',
+			'explore.removedFromWatchlist' => 'Removed from watchlist',
 			'explore.watchlistUpdateFailed' => 'Couldn\'t update watchlist',
+			'explore.watchlistNoMatch' => 'Couldn\'t match this item to a watchlist',
 			'explore.notInLibrary' => 'Not in your library',
 			'explore.inTheseLibraries' => 'In these libraries',
 			'explore.checkingLibrary' => 'Checking your library...',
@@ -7102,11 +7114,11 @@ extension on Translations {
 			'explore.stats.viewersWeek' => ({required Object n}) => '${n} watched this week',
 			'explore.stats.viewersMonth' => ({required Object n}) => '${n} watched this month',
 			'explore.stats.viewersYear' => ({required Object n}) => '${n} watched this year',
+			_ => null,
+		} ?? switch (path) {
 			'explore.stats.viewersAllTime' => ({required Object n}) => '${n} viewers',
 			'explore.stats.planning' => ({required Object n}) => '${n} planning to watch',
 			'explore.stats.favorited' => ({required Object n}) => '${n} favorites',
-			_ => null,
-		} ?? switch (path) {
 			'explore.stats.dropRate' => ({required Object percent}) => '${percent} dropped it',
 			'explore.stats.comments' => ({required num n}) => (_root.$meta.cardinalResolver ?? PluralResolvers.cardinal('en'))(n, one: '${n} comment', other: '${n} comments', ), 
 			'explore.stats.votes' => ({required Object n}) => '${n} votes',
@@ -7616,11 +7628,11 @@ extension on Translations {
 			'performanceOverlay.player' => 'Player',
 			'performanceOverlay.memory' => 'Memory',
 			'performanceOverlay.uiFps' => 'UI FPS',
+			_ => null,
+		} ?? switch (path) {
 			'externalPlayer.title' => 'External Player',
 			'externalPlayer.useExternalPlayer' => 'Use External Player',
 			'externalPlayer.useExternalPlayerDescription' => 'Open videos in another app',
-			_ => null,
-		} ?? switch (path) {
 			'externalPlayer.selectPlayer' => 'Select Player',
 			'externalPlayer.customPlayers' => 'Custom Players',
 			'externalPlayer.systemDefault' => 'System Default',