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/download.ts

@@ -10,7 +10,8 @@ import {
   getFfmpegPath,
   getFfprobePath,
   getSystem32Path,
-  getAppIconImage
+  getAppIconImage,
+  YTDLP_MISSING_MSG
 } from './binaries'
 import { getSettings, getDownloadArchivePath, getDefaultMediaDir } from './settings'
 import { ensureManagedYtdlp } from './ytdlp'
@@ -225,7 +226,7 @@ export function startDownload(wc: WebContents, opts: StartDownloadOptions): Star
   if (!existsSync(ytdlp)) {
     return {
       ok: false,
-      error: `yt-dlp.exe is missing and couldn't be restored from the bundle.\nReinstall AeroFetch, or drop yt-dlp.exe into resources/bin/ (see the README there).`
+      error: YTDLP_MISSING_MSG
     }
   }
   // ffmpeg is used by nearly every download (merge, audio extract, thumbnail/