Fix L46/L71/L74: unify yt-dlp missing message, editable dir inputs, swatch a11y

L46: Introduced YTDLP_MISSING_MSG constant in binaries.ts. All four callers
     (download, indexer, probe, ytdlp) now emit the same user-facing string:
     "yt-dlp.exe is missing. Open Settings -> Software update to re-download it."
     This removes newline-embedded messages and four divergent phrasings.

L71: Removed readOnly from videoDir/audioDir folder inputs in SettingsView.
     Added onChange handler so users can paste or type a path directly without
     being forced to use the Browse button. The path still flows through
     applySettings() in main, which validates it.

L74: Accent color swatches were triple-labeled (aria-pressed + aria-label +
     title). Replaced with a single aria-label that embeds "(selected)" state,
     plus title for the hover tooltip. Removed the incorrect aria-pressed
     (these are selection buttons, not toggles).

typecheck + 242 tests green.

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

src/main/binaries.ts

@@ -80,6 +80,10 @@ export function getFfprobePath(): string {
   return join(getBinDir(), 'ffprobe.exe')
 }
 
+/** User-facing error shown whenever yt-dlp.exe is not found at startup. */
+export const YTDLP_MISSING_MSG =
+  'yt-dlp.exe is missing. Open Settings → Software update to re-download it.'
+
 /** Optional bundled external downloader; absent unless dropped into resources/bin. */
 export function getAria2cPath(): string {
   return join(getBinDir(), 'aria2c.exe')