feat(web): radio queue auto-refreshes at 80% via ?exclude= param

Adds _radioSeedId state and a module-level $effect.root that triggers a
/api/radio?seed_track=…&exclude=… fetch when queue consumption reaches
80%, appending new tracks without resetting the radio seed. Clears seed
on any non-radio enqueue (playQueue/enqueueTrack/enqueueTracks). Five
new vitest cases cover the trigger threshold, append logic, in-flight
guard, and manual-enqueue reset path.

web/src/lib/player/store.svelte.ts

@@ -28,6 +28,12 @@ let _shuffle  = $state(false);
 let _repeat   = $state<RepeatMode>('off');
 let _error    = $state<string | null>(null);
 
+// M4c: track when the queue was seeded by a radio call so we can
+// auto-refresh at 80% consumption. Cleared when the user enqueues
+// from a non-radio source.
+let _radioSeedId          = $state<string | null>(null);
+let _radioRefreshInFlight = false;
+
 let _audioEl: HTMLAudioElement | null = null;
 
 export const player = {
@@ -49,6 +55,7 @@ export function registerAudioEl(el: HTMLAudioElement | null): void {
 }
 
 export function playQueue(tracks: TrackRef[], startIndex = 0): void {
+  _radioSeedId = null;  // M4c: non-radio enqueue clears the radio refresh state
   _queue = tracks;
   if (tracks.length === 0) {
     _index = 0;
@@ -199,11 +206,13 @@ export function reportStateFromAudio(
 }
 
 export function enqueueTrack(t: TrackRef): void {
+  _radioSeedId = null;  // M4c
   _queue = [..._queue, t];
   if (_state === 'idle') _index = 0;
 }
 
 export function enqueueTracks(ts: TrackRef[]): void {
+  _radioSeedId = null;  // M4c
   if (ts.length === 0) return;
   _queue = [..._queue, ...ts];
   if (_state === 'idle') _index = 0;
@@ -213,5 +222,44 @@ export async function playRadio(seedTrackId: string): Promise<void> {
   const resp = await api.get<RadioResponse>(
     `/api/radio?seed_track=${encodeURIComponent(seedTrackId)}`
   );
-  if (resp.tracks.length > 0) playQueue(resp.tracks, 0);
+  if (resp.tracks.length === 0) return;
+  playQueue(resp.tracks, 0);
+  _radioSeedId = seedTrackId;  // M4c: set AFTER playQueue (which clears it)
 }
+
+// M4c: auto-refresh radio queue when 80% consumed.
+// Fires whenever the consumption ratio crosses 80% AND the queue was
+// seeded by playRadio() AND no refresh is currently in-flight.
+$effect.root(() => {
+  $effect(() => {
+    if (_radioSeedId === null) return;
+    if (_radioRefreshInFlight) return;
+    if (_queue.length === 0) return;
+    const consumedRatio = (_index + 1) / _queue.length;
+    if (consumedRatio < 0.8) return;
+
+    _radioRefreshInFlight = true;
+    const seed = _radioSeedId;
+    const exclude = _queue.map((t) => t.id).join(',');
+    api
+      .get<RadioResponse>(
+        `/api/radio?seed_track=${encodeURIComponent(seed)}&exclude=${exclude}`
+      )
+      .then((resp) => {
+        // Strip the seed at index 0 (already in our queue) and any other
+        // tracks that happen to match the original seed id.
+        const newTracks = resp.tracks.slice(1).filter((t) => t.id !== seed);
+        if (newTracks.length > 0) {
+          // Append directly without calling enqueueTracks — that would
+          // clear _radioSeedId and prevent further refreshes.
+          _queue = [..._queue, ...newTracks];
+        }
+      })
+      .catch(() => {
+        // Swallow; next track-advance can retry.
+      })
+      .finally(() => {
+        _radioRefreshInFlight = false;
+      });
+  });
+});