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/components/AppHeader.svelte

@@ -1,19 +1,22 @@
 <!--
   Shared application header. Lifted out of the library layout so the account
   menu (and desktop nav) are available on every authenticated, non-immersive
-  screen, not only under /library. Routes that need in-header search (the
-  library layout) pass it in via the `search` snippet; other routes omit it.
+  screen, not only under /library.
+
+  The search box is owned here rather than passed in by a layout, so the same
+  bar renders on the library routes and on /search — searching from the header
+  no longer hands you to a screen with a different input.
 
   TRACES: UR-054 | DR-076
 -->
 <script lang="ts">
-  import type { Snippet } from "svelte";
   import { page } from "$app/stores";
   import AccountMenu from "$lib/components/account/AccountMenu.svelte";
-
-  let { search }: { search?: Snippet } = $props();
+  import HeaderSearch from "$lib/components/search/HeaderSearch.svelte";
+  import { showHeaderSearch } from "$lib/utils/layoutShell";
 
   const pathname = $derived($page.url.pathname);
+  const withSearch = $derived(showHeaderSearch({ pathname }));
 </script>
 
 <header class="sticky top-0 z-50 bg-[var(--color-background)]/95 backdrop-blur border-b border-gray-800 flex-shrink-0">
@@ -51,10 +54,10 @@
       </a>
     </nav>
 
-    <!-- Optional in-header search (library layout supplies it). -->
-    {#if search}
-      <div class="flex-1 max-w-md hidden md:block space-y-2">
-        {@render search()}
+    <!-- In-header search: the single md+ search input (library + /search). -->
+    {#if withSearch}
+      <div class="flex-1 max-w-md hidden md:block">
+        <HeaderSearch />
       </div>
     {/if}