feat(offline): play downloaded video, and drain the offline sync queue (0.4.6)

Bundles this session's work plus the concurrent search/offline/player changes.
Every gate passes on the combined tree: 885 frontend tests, 610 Rust tests,
clippy clean, boundary clean, trace coverage 86%.

Offline video playback — four separate defects, each of which alone stopped it:

  DR-133  A completed download's file_path is already absolute (the worker
          rewrites it on completion), but the player rooted it a second time and
          handed the webview /data/user/0/app//data/user/0/app/videos/x.mp4.
  DR-134  The asset protocol was never enabled: no protocol-asset feature and no
          assetProtocol config, so convertFileSrc produced URLs nothing answered.
          Also silently defeated the cached-thumbnail path, which fails soft to
          the server copy and hid it whenever the server was reachable.
  DR-137  Tauri's asset protocol answers a range-less request by reading the
          whole file into memory, and only advertises Accept-Ranges from inside
          its range branch, so the first request never learns ranges exist.
          Chromium gave up with PIPELINE_ERROR_READ after ~31s. Local media is
          now served by a loopback HTTP server: bounded 4 MiB chunks streamed
          from the file handle, every response length-delimited, and a range-less
          request answered with one chunk rather than the file. Confined by a
          per-session token and to the app data directory, because loopback is
          shared between apps on Android.
  DR-138  Release builds set usesCleartextTraffic=false, so Android rejected the
          request to that server before any I/O. A network-security-config
          exempts 127.0.0.1 only; a remote server must still be HTTPS.

Downloads:

  DR-135  download_item never records media_type and the reconnect resolver read
          that NULL as 'audio', so a movie queued from a media card had its URL
          resolved by get_audio_stream_url and completed as an audio-only
          transcode. The item's own type now decides.
  DR-136  Rows already downloaded that way are requeued on reconnect, since
          prevention alone leaves them reading "downloaded" and still unplayable.

Known limitation: a download taken at `original` quality is a byte copy of the
source, so it can be any container. One such file is an AVI holding XVID, which
the webview cannot play in any case — the media server serves it correctly and
Chromium refuses it. That needs either a transcoded download preset or the
native ExoPlayer surface work, and is not addressed here.

Also fixes two ID collisions between concurrent work: DR-143 defined twice
(search vs offline gate) and UT-131 defined twice (Episode Focus hero vs channel
cap). The search requirement is now DR-147 and the channel-cap test UT-141, with
their code references and matrix rows updated.

src/lib/utils/searchScope.ts

@@ -71,6 +71,16 @@ export function searchRouteUrl(query: string, scope: SearchScope): string {
   return `/search?${params.toString().replace(/\+/g, "%20")}`;
 }
 
+/** Normalise a pathname for comparison: drop query, hash and trailing slashes. */
+function normalizePath(pathname: string): string {
+  return pathname.split(/[?#]/)[0].replace(/\/+$/, "") || "/";
+}
+
+/** Whether `pathname` is the search surface itself. */
+export function isSearchRoute(pathname: string): boolean {
+  return normalizePath(pathname) === "/search";
+}
+
 /**
  * Whether a search typed on `pathname` must navigate to `/search` to be seen.
  *
@@ -82,8 +92,48 @@ export function searchRouteUrl(query: string, scope: SearchScope): string {
  */
 export function shouldNavigateToSearch(pathname: string, query: string): boolean {
   if (!query.trim()) return false;
-  const path = pathname.split(/[?#]/)[0].replace(/\/+$/, "") || "/";
-  return path !== "/search";
+  return !isSearchRoute(pathname);
+}
+
+/** Read a `?scope=` value, falling back when it is absent or unrecognised. */
+export function parseSearchScope(
+  raw: string | null | undefined,
+  fallback: SearchScope = "all"
+): SearchScope {
+  return SEARCH_SCOPES.includes(raw as SearchScope) ? (raw as SearchScope) : fallback;
+}
+
+/** The query + scope a `/search` URL asks the page to show. */
+export interface SearchSeed {
+  query: string;
+  scope: SearchScope;
+}
+
+/**
+ * What a `/search` URL should seed the page with, or `null` if it asks for
+ * nothing new.
+ *
+ * The URL is *consumed once per value*, not continuously reconciled against the
+ * live input. `applied` is the seed the caller last took from the URL: while it
+ * still matches, the user's own typing and chip picks govern, and only a real
+ * navigation (the header search sending a new query) re-seeds the page.
+ *
+ * Reconciling instead of consuming was the bug this replaced — the old effect
+ * compared the URL against `library.searchQuery`, so every keystroke's search
+ * re-ran it and snapped the input back to the query the header had sent.
+ *
+ * TRACES: UR-049 | DR-063, DR-064
+ */
+export function seedFromSearchUrl(
+  params: URLSearchParams,
+  applied: SearchSeed | null
+): SearchSeed | null {
+  const seed: SearchSeed = {
+    query: params.get("q") ?? "",
+    scope: parseSearchScope(params.get("scope")),
+  };
+  if (applied && applied.query === seed.query && applied.scope === seed.scope) return null;
+  return seed;
 }
 
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