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/renderer/src/components/SettingsView.tsx

@@ -485,11 +485,11 @@ export function SettingsView(): React.JSX.Element {
         >
           <div className={styles.folderRow}>
             <Input
-              readOnly
               className={styles.folderInput}
               value={videoDir}
               placeholder="Documents\Video (default)"
               contentBefore={<FolderRegular />}
+              onChange={(_, d) => update({ videoDir: d.value })}
             />
             <Button icon={<FolderRegular />} onClick={() => chooseDir('videoDir')}>
               Browse
@@ -508,11 +508,11 @@ export function SettingsView(): React.JSX.Element {
         >
           <div className={styles.folderRow}>
             <Input
-              readOnly
               className={styles.folderInput}
               value={audioDir}
               placeholder="Documents\Audio (default)"
               contentBefore={<FolderRegular />}
+              onChange={(_, d) => update({ audioDir: d.value })}
             />
             <Button icon={<FolderRegular />} onClick={() => chooseDir('audioDir')}>
               Browse
@@ -625,8 +625,7 @@ export function SettingsView(): React.JSX.Element {
                 )}
                 style={{ backgroundColor: opt.swatch }}
                 onClick={() => update({ accentColor: opt.value as AccentColor })}
-                aria-pressed={accentColor === opt.value}
-                aria-label={opt.label}
+                aria-label={`${opt.label}${accentColor === opt.value ? ' (selected)' : ''}`}
                 title={opt.label}
               />
             ))}