fix: surface playback errors and stabilize disk cache size

Playback errors in the global overlay (direct-start from Browse/Category/
StreamList screens) were silently discarded — the stream disappeared with
no user feedback. Now shows an error overlay with the streamlink message
and a dismiss hint; hls.js fatal errors use the same path.

Chromium computes disk-cache-size dynamically from free disk space, so
the value differs between runs. The cache index then contains a size that
no longer matches, producing "Invalid cache (current) size" on shutdown.
A fixed 500 MB cap keeps the value stable across restarts.

The cache classification thresholds (500 MB / 1 GB) were designed for an
uncapped cache. With a fixed 500 MB limit, the cache can never enter those
states, leaving dead UI states and unused logic behind.

src/main/index.ts

@@ -12,6 +12,9 @@ const isDev = !app.isPackaged
 // Erlaubt video.play() ohne vorangehende User-Geste (nötig, weil play() in hls.js-Callbacks
 // aufgerufen wird, die asynchron weit nach dem ursprünglichen Click-Event stattfinden).
 app.commandLine.appendSwitch('autoplay-policy', 'no-user-gesture-required')
+// Explizite Cache-Größe verhindert Chromiums dynamische Größenberechnung, die bei
+// wechselndem freiem Speicherplatz zu "Invalid cache (current) size"-Fehlern führt.
+app.commandLine.appendSwitch('disk-cache-size', String(500 * 1024 * 1024))
 
 function createWindow(): BrowserWindow {
   const isLinux = process.platform === 'linux'