M19: Move sidebarCollapsed from localStorage to electron-store

Sidebar collapsed state was an orphan in localStorage -- not backed up,
lost on portable installs, and inconsistent with every other preference.

Added sidebarCollapsed: boolean to the Settings type and DEFAULT_SETTINGS,
wired it through applySettings' boolean branch, and updated the renderer
store / main.tsx mock defaults. App.tsx now drives the toggle via
updateSettings({ sidebarCollapsed }) instead of localStorage.

typecheck + 242 tests + eslint green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CODE-AUDIT.md

@@ -277,7 +277,7 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
 - [ ] **M18 — Audio "quality" presets hard-code MP3.** `QUALITY_OPTIONS.audio` labels ("Best
   (MP3)", "320 kbps") are shown even when `downloadOptions.audioFormat` is opus/flac/wav, so the
   label contradicts the actual output format; `--audio-quality` is also meaningless for lossless.
-- [ ] **M19 — Sidebar-collapsed pref uses `localStorage`** while every other preference uses
+- [x] **M19 — Sidebar-collapsed pref uses `localStorage`** while every other preference uses
   electron-store — won't back up/restore and is lost in the portable build across machines.
 - [ ] **M20 — ProgressBar / native `<select>` accessibility.** QueueItem/DownloadsView
   `ProgressBar`s have no accessible name; several `Select`s carry both a `<Field label>` and an