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/probe.ts

@@ -1,6 +1,6 @@
 import { execFile } from 'child_process'
 import { existsSync } from 'fs'
-import { getYtdlpPath } from './binaries'
+import { getYtdlpPath, YTDLP_MISSING_MSG } from './binaries'
 import { fmtBytes } from './lib/formatters'
 import { cleanError } from './log'
 import { assertHttpUrl } from './url'
@@ -115,7 +115,7 @@ export function probeMedia(url: string): Promise<ProbeResult> {
   if (!existsSync(ytdlp)) {
     return Promise.resolve({
       ok: false,
-      error: `yt-dlp.exe not found at ${ytdlp}\nDrop it into resources/bin/ (see the README there).`
+      error: YTDLP_MISSING_MSG
     })
   }
   let target: string