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/download/events.rs

@@ -20,6 +20,10 @@ pub enum DownloadEvent {
         bytes_downloaded: i64,
         total_bytes: Option<i64>,
         progress: f64, // 0.0 to 1.0
+        /// `total_bytes` is a prediction rather than the server's
+        /// `Content-Length`, so `progress` stops short of 1.0 until the
+        /// download completes. TRACES: UR-071 | DR-290
+        estimated: bool,
     },
     /// Download completed successfully
     #[serde(rename_all = "camelCase")]
@@ -27,6 +31,10 @@ pub enum DownloadEvent {
         download_id: i64,
         item_id: String,
         file_path: String,
+        /// Bytes actually written. The frontend persists completion too, and
+        /// without this it fell back to the row's `file_size` — which is a
+        /// prediction for a transcode (DR-290), not the real size.
+        bytes_downloaded: i64,
     },
     /// Download failed with error
     #[serde(rename_all = "camelCase")]
@@ -60,6 +68,7 @@ mod tests {
             bytes_downloaded: 1024,
             total_bytes: Some(2048),
             progress: 0.5,
+            estimated: false,
         };
 
         let json = serde_json::to_string(&event).unwrap();
@@ -86,6 +95,7 @@ mod tests {
             download_id: 42,
             item_id: "song456".to_string(),
             file_path: "/path/to/file.mp3".to_string(),
+            bytes_downloaded: 4096,
         };
 
         let json = serde_json::to_string(&event).unwrap();