Resume tracking: per-episode positions, player-agnostic fallback, series continue point

- Fix merged playback-state dropping season/episode, which broke series
  resume restore entirely.
- Track watch progress per episode (episodes map keyed S..E.., with done
  markers on completion) while keeping one continue point per series as
  the last watched episode, used for spoiler protection and season
  selection. MPC-BE tracker resumes from each episode's own position.
- Assumed-playback fallback for any player and server-only mode: a launch
  starts a session and the guessed position (base + elapsed, capped at
  runtime) is written when frontend activity resumes; the real MPC-BE
  tracker overrides guesses via timestamps. Frontend reports input
  activity (throttled) via POST /api/activity.
- Ignore watches under 5 minutes in both trackers.
- Episode tiles show a small quadrant-circle watch indicator; near end
  counts as fully watched, no data shows nothing.
- Fix episode audio ownership: single audioOwnerKey shared by mouse hover
  and keyboard/gamepad focus, no longer clobbered by playback sync;
  idle fade can be re-armed by continued activity.

frontend/src/api.ts

@@ -148,10 +148,25 @@ function splitAssetTypePath(assetPath: string): { assetType: string; relativePat
   return { assetType: assetType.toLowerCase(), relativePath: rest.join("/") }
 }
 
+/** Watch progress for one episode of a series. */
+export interface EpisodeWatchEntry {
+  pos: number
+  done: boolean
+}
+
+/** One stored continue point. season/episode are set for series, null for movies. */
+export interface ResumePositionEntry {
+  pos: number
+  season: number | null
+  episode: number | null
+  /** Per-episode watch progress for series, keyed "S<season>E<episode>". */
+  episodes?: Record<string, EpisodeWatchEntry>
+}
+
 /**
  * Fetch merged resume positions from all roots.
  */
-export async function fetchResumePositions(): Promise<Record<string, number>> {
+export async function fetchResumePositions(): Promise<Record<string, ResumePositionEntry>> {
   try {
     const response = await fetch("/api/meta/playback-state")
     if (!response.ok) return {}
@@ -160,12 +175,32 @@ export async function fetchResumePositions(): Promise<Record<string, number>> {
     if (!positions || typeof positions !== "object") {
       return {}
     }
-    const normalized: Record<string, number> = {}
+    const normalized: Record<string, ResumePositionEntry> = {}
     for (const [slug, value] of Object.entries(positions as Record<string, unknown>)) {
       if (!value || typeof value !== "object") continue
-      const pos = (value as { pos?: unknown }).pos
-      if (typeof pos === "number" && Number.isFinite(pos) && pos > 0) {
-        normalized[slug] = pos
+      const entry = value as { pos?: unknown; season?: unknown; episode?: unknown }
+      if (typeof entry.pos !== "number" || !Number.isFinite(entry.pos) || entry.pos < 0) {
+        continue
+      }
+      normalized[slug] = {
+        pos: entry.pos,
+        season: typeof entry.season === "number" ? entry.season : null,
+        episode: typeof entry.episode === "number" ? entry.episode : null,
+      }
+      const rawEpisodes = (entry as { episodes?: unknown }).episodes
+      if (rawEpisodes && typeof rawEpisodes === "object") {
+        const watches: Record<string, EpisodeWatchEntry> = {}
+        for (const [key, watch] of Object.entries(
+          rawEpisodes as Record<string, unknown>,
+        )) {
+          if (!watch || typeof watch !== "object") continue
+          const w = watch as { pos?: unknown; done?: unknown }
+          if (typeof w.pos !== "number" || !Number.isFinite(w.pos)) continue
+          watches[key] = { pos: w.pos, done: w.done === true }
+        }
+        if (Object.keys(watches).length > 0) {
+          normalized[slug].episodes = watches
+        }
       }
     }
     return normalized
@@ -174,6 +209,29 @@ export async function fetchResumePositions(): Promise<Record<string, number>> {
   }
 }
 
+/**
+ * Report that the user is actively interacting with the UI.
+ *
+ * Ends any server-side assumed-playback session (launched item is assumed
+ * watched while the UI sees no input). Throttled; fire-and-forget.
+ */
+let lastActivityReportAt = 0
+export function reportUserActivity(): void {
+  const now = Date.now()
+  if (now - lastActivityReportAt < 5000) return
+  lastActivityReportAt = now
+  void fetch("/api/activity", { method: "POST" })
+    .then(async (response) => {
+      if (!response.ok) return
+      const data = await response.json().catch(() => null)
+      if (data?.finalized) {
+        // An assumed-playback position was just written; let views refetch.
+        window.dispatchEvent(new Event("mediahive:resume-updated"))
+      }
+    })
+    .catch(() => {})
+}
+
 /**
  * Replace the full root set atomically
  */