feat(settings): global Advanced Mode toggle + playlist page layout (#556) (#720)

Adds a per-element visibility toggle for the playlist detail page (Add
Songs, Import CSV, Download ZIP, Cache Offline, Suggestions) and reworks
the way uncommon options are surfaced: instead of a per-tab collapsible
group, a global "Advanced" toggle in the Settings header reveals all
`advanced` sub-sections across every tab and marks each one with a small
badge. Sets the pattern up so any future advanced option lives in its
natural tab, gated by the same switch.

- New `advancedSettingsEnabled` boolean on `authStore`
  (UiAppearance slice, persisted with the rest of the store).
- `SettingsSubSection` gains an `advanced?: boolean` prop. Hidden when
  the toggle is off; renders an "Advanced" pill in the header when on.
- Settings header gets a Toggle-Switch next to the search lupe.
- `PersonalisationTab` flattens — Sidebar + Home stay always visible;
  Artist sections, Queue Toolbar, and the new Playlist layout get
  `advanced` and disappear by default. `PersonalisationAdvancedGroup`
  component + CSS removed.
- New `playlistLayoutStore` (Zustand + persist, items[{id,visible}] +
  rehydrate sanitize) following the queueToolbarStore pattern.
- `PlaylistHero` and `PlaylistSuggestions` gate the four toolbar buttons
  and the suggestions rail on the store directly.
- One-time migration in MainApp on mount: if the user had opened the
  old per-tab Advanced group (`psysonic_personalisation_advanced_open
  === 'true'`) OR already customised any of the three sub-sections,
  Advanced Mode auto-enables on first launch. Idempotent via a
  localStorage flag; legacy key removed afterwards.
- New i18n keys `settings.advancedMode`, `settings.advancedModeTooltip`,
  `settings.advancedBadge`, `settings.playlistLayout*` in all 9 locales.

Reuses kveld9's design from PR #556; not merged because the locale split
landed afterwards. Credited under the existing kveld9 entry in
settingsCredits.ts.

Co-authored-by: Kveld. <kveld912@proton.me>

src/config/settingsCredits.ts

@@ -168,6 +168,7 @@ const CONTRIBUTOR_ENTRIES = [
       'Queue Panel — position counter, tri-state duration toggle (total/remaining/ETA), persistent Now Playing collapse, animated EQ indicator (PR #419)',
       'Community themes — redesign pass: removed 5 overlapping / eye-straining palettes, added 8 new dark themes (Obsidian Black, Carbon Grey, Volcanic Dark, Forest Green, Violet Haze, Copper Oxide, Sakura Night, Obsidian Gold) (PR #490)',
       'Customizable queue toolbar — drag-and-drop button reordering, per-button visibility toggles, optional separator, persisted across restarts (PR #534)',
+      'Playlist page layout — per-element visibility toggles (Add Songs, Import CSV, Download ZIP, Cache Offline, Suggestions) under Settings → Personalisation (PR #556)',
     ],
   },
   {