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/lib/stores/sessions.ts

@@ -89,7 +89,7 @@ function createSessionsStore() {
   /**
    * Send play/pause toggle command
    */
-  async function sendPlayPause(sessionId: string): Promise<void> {
+  async function sendPlayPause(sessionId: string | null | undefined): Promise<void> {
     try {
       await invoke("remote_send_command", {
         sessionId,
@@ -106,7 +106,7 @@ function createSessionsStore() {
   /**
    * Send stop command
    */
-  async function sendStop(sessionId: string): Promise<void> {
+  async function sendStop(sessionId: string | null | undefined): Promise<void> {
     try {
       await invoke("remote_send_command", {
         sessionId,
@@ -122,7 +122,7 @@ function createSessionsStore() {
   /**
    * Send next track command
    */
-  async function sendNext(sessionId: string): Promise<void> {
+  async function sendNext(sessionId: string | null | undefined): Promise<void> {
     try {
       await invoke("remote_send_command", {
         sessionId,
@@ -138,7 +138,7 @@ function createSessionsStore() {
   /**
    * Send previous track command
    */
-  async function sendPrevious(sessionId: string): Promise<void> {
+  async function sendPrevious(sessionId: string | null | undefined): Promise<void> {
     try {
       await invoke("remote_send_command", {
         sessionId,
@@ -154,7 +154,7 @@ function createSessionsStore() {
   /**
    * Seek to position (in ticks)
    */
-  async function sendSeek(sessionId: string, positionTicks: number): Promise<void> {
+  async function sendSeek(sessionId: string | null | undefined, positionTicks: number): Promise<void> {
     try {
       await invoke("remote_session_seek", {
         sessionId,
@@ -170,7 +170,7 @@ function createSessionsStore() {
   /**
    * Set volume (0-100)
    */
-  async function sendVolume(sessionId: string, volume: number): Promise<void> {
+  async function sendVolume(sessionId: string | null | undefined, volume: number): Promise<void> {
     try {
       await invoke("remote_session_set_volume", {
         sessionId,
@@ -186,7 +186,7 @@ function createSessionsStore() {
   /**
    * Toggle mute
    */
-  async function sendToggleMute(sessionId: string): Promise<void> {
+  async function sendToggleMute(sessionId: string | null | undefined): Promise<void> {
     try {
       await invoke("remote_send_command", {
         sessionId,
@@ -203,7 +203,7 @@ function createSessionsStore() {
    * Play item(s) on remote session
    */
   async function playOnSession(
-    sessionId: string,
+    sessionId: string | null | undefined,
     itemIds: string[],
     startIndex = 0
   ): Promise<void> {