feat(downloads): estimate a transcode's size so the progress bar moves

A transcode is produced as it is sent — chunked, with no Content-Length —
and the worker reported progress 0.0 for its whole duration: an empty bar
reading "0%" while the byte count climbed for an hour. That is the case
every film whose audio must be re-encoded lands in.

The backend already fetches the item to decide the audio policy, and that
item carries what a prediction needs: the source's size (an `original`
download copies the picture, so the output is the source give or take the
audio track) and its runtime (a preset re-encodes at fixed rates, so the
size is rate × runtime — from a preset table the URL builder now shares, so
the two cannot drift). The prediction is made where the URL is resolved and
persisted as the row's file_size. The worker uses it only when the response
has no length; the server's figure always wins; an estimated bar is capped
at 99% so a low prediction never shows a finished download still running;
and the Completed event now carries the bytes actually written so the
frontend stops persisting the row's file_size as the final size.

The row renders three honest states: exact "42%", estimated "~42%" with
"X / ~Y", or — with no total at all — an indeterminate band and the bytes
so far, never "0%". The single-video button joins the series/season buttons
on the enqueue path so all three resolve, and predict, in one place.

DR-290, UT-252, UT-253, UT-254.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src-tauri/src/repository/mod.rs

@@ -213,7 +213,7 @@ pub trait MediaRepository: Send + Sync {
     /// [`resolve_video_download_url`] rather than calling it directly.
     ///
     /// `source_audio_codec` is the codec of the audio track the server would
-    /// serve (see [`served_audio_codec`]); `None` when it is not known. At
+    /// serve (see [`resolve_video_download`]); `None` when it is not known. At
     /// `original` quality it decides whether the file can be copied byte-for-byte
     /// or has to have its audio re-encoded on the way down — a downloaded file is
     /// played back with no server in reach, so it has to be decodable *here*.
@@ -345,35 +345,70 @@ pub trait MediaRepository: Send + Sync {
     ) -> Result<(), RepoError>;
 }
 
-/// The audio codec the server would serve for `item_id` — the default track, or
-/// the first when none is marked, matching the track Jellyfin picks.
+/// A video download, resolved: the URL to fetch and, where the item told us
+/// enough, how many bytes to expect from it.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ResolvedVideoDownload {
+    pub url: String,
+    /// Predicted size (see `download::estimate`), used as the progress total
+    /// when the response carries no `Content-Length` — a transcode never does.
+    pub expected_bytes: Option<u64>,
+}
+
+/// Resolve the download URL for a video, applying the audio-codec policy that
+/// keeps the saved file playable offline (DR-171), and predict its size from
+/// the same item lookup (DR-290).
 ///
-/// `None` when the item has no audio, names no codec, or cannot be fetched. A
-/// caller must read that as "unknown", never as "fine": it is the input to a
-/// policy that only *adds* a transcode, so an unknown codec leaves behaviour
-/// exactly as it was.
+/// Every video download goes through here rather than calling the builder
+/// directly: the builder is pure and cannot look the codec up, and a caller that
+/// forgets to is exactly how the silent downloads shipped.
 ///
-/// TRACES: UR-071 | DR-171 | UT-166
-pub async fn served_audio_codec(repo: &dyn MediaRepository, item_id: &str) -> Option<String> {
-    let item = repo.get_item(item_id).await.ok()?;
+/// TRACES: UR-071 | DR-171, DR-290
+pub async fn resolve_video_download(
+    repo: &dyn MediaRepository,
+    item_id: &str,
+    quality: &str,
+    media_source_id: Option<&str>,
+) -> ResolvedVideoDownload {
+    let item = repo.get_item(item_id).await.ok();
+
     let audio: Vec<(Option<&str>, bool)> = item
-        .media_streams
-        .as_deref()
+        .as_ref()
+        .and_then(|i| i.media_streams.as_deref())
         .unwrap_or_default()
         .iter()
         .filter(|s| s.stream_type == "Audio")
         .map(|s| (s.codec.as_deref(), s.is_default))
         .collect();
+    // The default track, or the first when none is marked, matching the track
+    // Jellyfin picks. `None` reads as "unknown", never as "fine": it feeds a
+    // policy that only *adds* a transcode, so an unknown codec leaves behaviour
+    // exactly as it was. TRACES: UR-071 | DR-171 | UT-166
+    let codec = device_profile::served_audio_codec(&audio);
 
-    device_profile::served_audio_codec(&audio).map(str::to_string)
+    // The source that will be served: the one asked for, else the first —
+    // the same choice Jellyfin makes when no `mediaSourceId` is given.
+    let source_size = item.as_ref().and_then(|i| {
+        let sources = i.media_sources.as_deref()?;
+        let source = match media_source_id {
+            Some(id) => sources.iter().find(|s| s.id == id),
+            None => sources.first(),
+        };
+        source?.size
+    });
+    let expected_bytes = crate::download::estimate::expected_download_bytes(
+        quality,
+        item.as_ref().and_then(|i| i.runtime_ticks),
+        source_size,
+    );
+
+    ResolvedVideoDownload {
+        url: repo.get_video_download_url(item_id, quality, media_source_id, codec),
+        expected_bytes,
+    }
 }
 
-/// Resolve the download URL for a video, applying the audio-codec policy that
-/// keeps the saved file playable offline (DR-171).
-///
-/// Every video download goes through here rather than calling the builder
-/// directly: the builder is pure and cannot look the codec up, and a caller that
-/// forgets to is exactly how the silent downloads shipped.
+/// [`resolve_video_download`] for callers that only need the URL.
 ///
 /// TRACES: UR-071 | DR-171
 pub async fn resolve_video_download_url(
@@ -382,6 +417,7 @@ pub async fn resolve_video_download_url(
     quality: &str,
     media_source_id: Option<&str>,
 ) -> String {
-    let codec = served_audio_codec(repo, item_id).await;
-    repo.get_video_download_url(item_id, quality, media_source_id, codec.as_deref())
+    resolve_video_download(repo, item_id, quality, media_source_id)
+        .await
+        .url
 }