E frontend reconciliation (2/2): align consumers to generated bindings; svelte-check clean

Reconcile the ~50 frontend files that consumed the old hand-written types against
the stricter generated bindings (141 -> 0 svelte-check errors):

- Nullable strictness: null-coalesce/guard at consumer sites; widen shared helpers
  (CachedImage.tag, ticksToSeconds, formatDuration, getSessionIcon, session store
  send*/transferToRemote) to accept null.
- Field-name fixes exposed by codegen: userData.played -> isPlayed; remote
  NowPlayingItem uses album (not albumName); ArtistItem id/name.
- player.ts: normalize remote NowPlayingItem -> MediaItem in mergedMedia.
- ArtistItem now serializes camelCase (PascalCase alias retained for Jellyfin
  deserialize); update its serialize test.
- Update downloads.test.ts to the bundled { request } shape; complete the
  sessions.test.ts NowPlayingItem mock.

Frontend: svelte-check 0 errors, vitest 448 passing. Backend: 387 passing.

src/routes/library/[id]/+page.svelte

@@ -456,7 +456,7 @@
         <div class="space-y-2">
           {#if item.people.some(p => p.type === "Director")}
             <CrewLinks
-              people={item.people}
+              people={item.people ?? undefined}
               roleFilter={["Director"]}
               label="Directed by"
               maxShow={3}
@@ -465,7 +465,7 @@
 
           {#if item.people.some(p => p.type === "Writer")}
             <CrewLinks
-              people={item.people}
+              people={item.people ?? undefined}
               roleFilter={["Writer"]}
               label="Written by"
               maxShow={3}
@@ -474,7 +474,7 @@
 
           {#if item.people.some(p => p.type === "Composer")}
             <CrewLinks
-              people={item.people}
+              people={item.people ?? undefined}
               roleFilter={["Composer"]}
               label="Music by"
               maxShow={2}
@@ -486,13 +486,13 @@
       <!-- Genre Tags -->
       {#if item.genres?.length}
         <div>
-          <GenreTags genres={item.genres} maxShow={6} />
+          <GenreTags genres={item.genres ?? undefined} maxShow={6} />
         </div>
       {/if}
 
       <!-- Cast Section - for Movies, Series, and Episodes -->
       {#if (item.type === "Movie" || item.type === "Series" || item.type === "Episode") && item.people?.length}
-        <CastSection people={item.people} />
+        <CastSection people={item.people ?? undefined} />
       {/if}
 
       <!-- Related Items Section - for Movies and Series -->
@@ -500,8 +500,8 @@
         <RelatedItemsSection
           currentItemId={item.id}
           itemType={item.type}
-          genres={item.genres}
-          people={item.people}
+          genres={item.genres ?? undefined}
+          people={item.people ?? undefined}
           limit={12}
         />
       {/if}
@@ -528,7 +528,7 @@
               <RelatedItemsSection
                 currentItemId={item.id}
                 itemType="MusicAlbum"
-                genres={item.genres}
+                genres={item.genres ?? undefined}
                 artistIds={item.artistItems?.map(a => a.id)}
                 limit={12}
               />