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>

CODE-AUDIT.md

@@ -469,7 +469,7 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
   but there's no ESLint/Prettier config, script, or dependency — no enforced style tooling.
 - [x] **L45 — Three self-descriptions** drift: pkg `description` "A yt-dlp frontend for Windows" vs
   builder `copyright` "yt-dlp frontend" vs Sidebar caption "yt-dlp frontend".
-- [ ] **L46 — yt-dlp-missing error copy differs across 4 modules** (download/ytdlp/probe/indexer):
+- [x] **L46 — yt-dlp-missing error copy differs across 4 modules** (download/ytdlp/probe/indexer):
   "Reinstall AeroFetch, or drop…" vs "Download it into resources/bin/…" vs "Drop it into resources/bin/."
 - [x] **L47 — `probeMeta` 30s timeout returns null silently** — no surfaced message, unlike every
   other yt-dlp timeout ("Timed out …").
@@ -510,12 +510,12 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
   (`typeof crypto !== 'undefined'` is always true) — dead defensive branch (see M7). *Fixed with M7: the
   branch now lives once in `newId` (kept so the fn is total in any host) and is covered by a test that stubs
   `crypto` to exercise it.*
-- [ ] **L71 — Settings folder inputs are `readOnly`** with only a Browse button — no way to paste a
+- [x] **L71 — Settings folder inputs are `readOnly`** with only a Browse button — no way to paste a
   known path.
 - [x] **L72 — History re-download drops the thumbnail** (passes only title/channel), so the
   re-queued row loses its thumbnail until re-probed.
 - [ ] **L73 — Two verbs for the same probe action** — DownloadBar "Fetch" vs LibraryView "Index".
-- [ ] **L74 — Accent swatches are triple-labeled** (`aria-pressed` + `aria-label` + `title`).
+- [x] **L74 — Accent swatches are triple-labeled** (`aria-pressed` + `aria-label` + `title`).
 - [x] **L75 — Update-token field always visible** in the Software-update card, even when no update
   is pending — buries an advanced/rarely-needed input.
 - [x] **L76 — Duplicate-warning may show a placeholder title** — `setDup(existing.title)` can be the