Fix H3/L30/L44/L60/L90/L25/L32 polish: dependencies, naming, consistency

L30: Remove redundant loading=lazy from MediaThumb images inside the
     virtualizer which only mounts visible rows anyway.
L44: electron-builder.yml now excludes the actual eslint.config.js and
     .prettierrc rather than non-existent legacy .eslintrc/.prettierrc.yaml.
L60: IpcChannels.externalUrl renamed from external-url to app:external-url
     to match every other channel name category prefix.
L90: settings.ts sanitizeOptions now uses (ARR as readonly string[]).includes
     pattern consistently with setSettings; removes the as-never hack.

typecheck + 242 tests + eslint + prettier green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/settings.ts

@@ -16,7 +16,10 @@ import {
   isYtdlpUpdateChannel,
   type Settings,
   type DownloadOptions,
-  type SponsorBlockCategory
+  type SponsorBlockCategory,
+  type AudioFormat,
+  type VideoContainer,
+  type VideoCodecPref
 } from '@shared/ipc'
 
 const DEFAULTS: Settings = {
@@ -121,12 +124,16 @@ function sanitizeOptions(input: unknown): DownloadOptions {
       ) as SponsorBlockCategory[])
     : d.sponsorBlockCategories
   return {
-    audioFormat: AUDIO_FORMATS.includes(o.audioFormat as never) ? o.audioFormat! : d.audioFormat,
-    videoContainer: VIDEO_CONTAINERS.includes(o.videoContainer as never)
-      ? o.videoContainer!
+    audioFormat: (AUDIO_FORMATS as readonly string[]).includes(o.audioFormat as string)
+      ? (o.audioFormat as AudioFormat)
+      : d.audioFormat,
+    videoContainer: (VIDEO_CONTAINERS as readonly string[]).includes(o.videoContainer as string)
+      ? (o.videoContainer as VideoContainer)
       : d.videoContainer,
-    preferredVideoCodec: VIDEO_CODECS.includes(o.preferredVideoCodec as never)
-      ? o.preferredVideoCodec!
+    preferredVideoCodec: (VIDEO_CODECS as readonly string[]).includes(
+      o.preferredVideoCodec as string
+    )
+      ? (o.preferredVideoCodec as VideoCodecPref)
       : d.preferredVideoCodec,
     formatSort: typeof o.formatSort === 'string' ? o.formatSort.trim() : d.formatSort,
     embedSubtitles: bool(o.embedSubtitles, d.embedSubtitles),