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/indexer.ts

@@ -11,7 +11,7 @@
 
 import { execFile } from 'child_process'
 import { existsSync } from 'fs'
-import { getYtdlpPath } from './binaries'
+import { getYtdlpPath, YTDLP_MISSING_MSG } from './binaries'
 import { cleanError } from './log'
 import { assertHttpUrl } from './url'
 import {
@@ -90,7 +90,7 @@ export async function indexSource(
     return { ok: false, error: (e as Error).message }
   }
   if (!existsSync(getYtdlpPath())) {
-    return { ok: false, error: 'yt-dlp.exe not found. Drop it into resources/bin/.' }
+    return { ok: false, error: YTDLP_MISSING_MSG }
   }
 
   const cls = classifySource(url)