fix(subsync): sync subtitles from stream URLs, auto-discover tool paths

- Download URL-loaded subtitle tracks (Aniyomi extension streams, Jellyfin) to a temp file first, reusing mpv's own request headers, instead of rejecting them with "Subtitle file not found"
- Pass mpv's headers through to ffmpeg for internal track extraction from streams too
- Auto-discover alass/ffsubsync/ffmpeg on PATH and common install prefixes when the config path is empty, instead of a hard-coded /usr/bin fallback that never exists on macOS
- Log subsync failures to the app log, not just a toast that vanishes in seconds
- Update docs-site and README credits for the new behavior

docs-site/configuration.md

@@ -1214,11 +1214,15 @@ Sync the active subtitle track from the overlay picker using `alass` or `ffsubsy
 
 | Option           | Values          | Description                                                                                                               |
 | ---------------- | --------------- | ------------------------------------------------------------------------------------------------------------------------- |
-| `alass_path`     | string path     | Path to `alass` executable. Empty falls back to `/usr/bin/alass`. `alass` must be installed separately.                   |
-| `ffsubsync_path` | string path     | Path to `ffsubsync` executable. Empty falls back to `/usr/bin/ffsubsync`. `ffsubsync` must be installed separately.       |
-| `ffmpeg_path`    | string path     | Path to `ffmpeg` (used for internal subtitle extraction). Empty or `null` falls back to `/usr/bin/ffmpeg`.                |
+| `alass_path`     | string path     | Path to `alass` executable. Empty auto-discovers `alass` or `alass-cli`. `alass` must be installed separately.            |
+| `ffsubsync_path` | string path     | Path to `ffsubsync` executable. Empty auto-discovers `ffsubsync`. `ffsubsync` must be installed separately.               |
+| `ffmpeg_path`    | string path     | Path to `ffmpeg` (used for internal subtitle extraction). Empty or `null` auto-discovers `ffmpeg`.                        |
 | `replace`        | `true`, `false` | When `true` (default), overwrite the active subtitle file on successful sync. When `false`, write `<name>_retimed.<ext>`. |
 
+Auto-discovery searches `PATH`, then the usual install prefixes (`/opt/homebrew/bin`, `/usr/local/bin`, `/opt/local/bin`, `/usr/bin`, `/bin`) — a GUI launch inherits a minimal `PATH` that often omits the first two. Set the option explicitly if your binary lives elsewhere.
+
+Subtitle tracks that mpv loaded from a URL (Aniyomi extension streams, Jellyfin) are downloaded to a temporary file first, reusing mpv's own request headers, so they can be used as either the sync target or the alass reference.
+
 Stats dashboard sentence mining also uses `alass_path` when available to align a local English sidecar against the local Japanese sidecar before filling the card translation field. This stats-only retime writes a temporary cached copy and never edits the original subtitle files.
 
 Default trigger is `Ctrl+Alt+S` via `shortcuts.triggerSubsync`.

docs-site/troubleshooting.md

@@ -207,21 +207,26 @@ Resume playback and wait for the next subtitle to appear, then try mining again.
 
 Both **alass** and **ffsubsync** are optional external dependencies. Subtitle syncing requires at least one of them to be installed.
 
-**"Configured alass executable not found"**
+Subsync writes to the application log under the `subsync` scope, so the full command failure — exit code, stderr, resolved file paths — is recorded there as well as on the OSD.
+
+**"Could not find alass" / "Configured alass executable not found"**
 
 Install alass or configure the path:
 
+- **Homebrew**: `brew install alass`
 - **Arch Linux (AUR)**: `paru -S alass`
 - **Cargo**: `cargo install alass-cli`
 - Set the path: `subsync.alass_path` in your config.
 
-**"Configured ffsubsync executable not found"**
+Leaving the option empty searches `PATH` plus the usual install prefixes, and accepts either `alass` or `alass-cli`. Set the option explicitly when the binary lives somewhere else. The second message means the configured path itself does not exist — SubMiner never silently substitutes a different binary for one you named.
+
+**"Could not find ffsubsync" / "Configured ffsubsync executable not found"**
 
 Install ffsubsync or configure the path:
 
 - **Arch Linux (AUR)**: `paru -S python-ffsubsync`
 - **pip**: `pip install ffsubsync`
-- Must be on `PATH` or configured via `subsync.ffsubsync_path` in your config.
+- Must be discoverable, or configured via `subsync.ffsubsync_path` in your config.
 
 **"alass synchronization failed" / "ffsubsync synchronization failed"**
 
@@ -232,6 +237,12 @@ If subtitle sync fails (the error message is prefixed with the engine name):
 - Try running the sync tool manually to see detailed error output.
 - ffsubsync requires local files and cannot handle remote media streams (e.g., streaming URLs).
 
+**Syncing subtitles on a stream (Aniyomi extensions, Jellyfin)**
+
+Subtitle tracks mpv loaded from a URL are downloaded to a temporary file first, reusing mpv's own request headers, so they work as either the sync target or the alass reference. Downloading a Japanese track from Jimaku or TsukiHime while a stream is playing also works — it becomes the primary track and therefore the sync target.
+
+Internal subtitle tracks of a stream still go through `ffmpeg`, which has to reach the origin itself. If that fails, prefer an external track or a Jimaku download as the reference.
+
 ## TsukiHime
 
 **"xz binary not found"**