fix(mainstage): rename Home Page setting, start-page fallback + empty state (#975)

* fix(mainstage): rename Home Page setting, add start-page fallback + empty state

- Rename "Home Page" personalisation section to "Mainstage" across 9 locales
  so the heading matches the sidebar entry; add "mainstage" search keyword.
- Index route "/" now redirects to the first visible library item when the
  Mainstage sidebar entry is hidden, instead of stranding the app on a blank
  page. New pure resolveStartRoute() mirrors sidebar order + nav-mode gating.
- Show a guided empty state on Mainstage when every section is toggled off,
  with a CTA into Settings -> Personalisation.
- Unit tests for resolveStartRoute.

* docs(changelog): Mainstage rename, start-page fallback + empty state (#975)

src/utils/componentHelpers/sidebarNavReorder.test.ts

@@ -0,0 +1,47 @@
+import { describe, expect, it } from 'vitest';
+import { resolveStartRoute } from './sidebarNavReorder';
+import { DEFAULT_SIDEBAR_ITEMS, type SidebarItemConfig } from '../../store/sidebarStore';
+
+const hide = (items: SidebarItemConfig[], ...ids: string[]): SidebarItemConfig[] =>
+  items.map(i => (ids.includes(i.id) ? { ...i, visible: false } : i));
+
+const only = (...ids: string[]): SidebarItemConfig[] =>
+  DEFAULT_SIDEBAR_ITEMS.map(i => ({ ...i, visible: ids.includes(i.id) }));
+
+describe('resolveStartRoute', () => {
+  it('falls back to the first visible library item when Mainstage is hidden', () => {
+    const items = hide(DEFAULT_SIDEBAR_ITEMS, 'mainstage');
+    expect(resolveStartRoute(items, 'hub', false)).toBe('/new-releases');
+  });
+
+  it('skips Mainstage ("/") even if it is still flagged visible', () => {
+    // Resolver is only consulted when Mainstage is hidden, but it must never
+    // hand back "/" — that would redirect the index route onto itself.
+    expect(resolveStartRoute(DEFAULT_SIDEBAR_ITEMS, 'hub', false)).toBe('/new-releases');
+  });
+
+  it('returns null when no library item is visible (caller renders empty Mainstage)', () => {
+    const items = DEFAULT_SIDEBAR_ITEMS.map(i => ({ ...i, visible: false }));
+    expect(resolveStartRoute(items, 'hub', false)).toBeNull();
+  });
+
+  it('honours sidebar order — first visible entry wins', () => {
+    expect(resolveStartRoute(only('favorites', 'artists'), 'hub', false)).toBe('/artists');
+    expect(resolveStartRoute(only('favorites'), 'hub', false)).toBe('/favorites');
+  });
+
+  it('skips luckyMix when it is not available', () => {
+    const items = only('luckyMix', 'genres');
+    // separate mode surfaces luckyMix, but availability gate is off → next item
+    expect(resolveStartRoute(items, 'separate', false)).toBe('/genres');
+    expect(resolveStartRoute(items, 'separate', true)).toBe('/lucky-mix');
+  });
+
+  it('respects randomNavMode hub/separate gating', () => {
+    // randomPicker (Build a Mix) only exists in hub mode; randomMix only in separate.
+    expect(resolveStartRoute(only('randomPicker'), 'hub', false)).toBe('/random');
+    expect(resolveStartRoute(only('randomPicker'), 'separate', false)).toBeNull();
+    expect(resolveStartRoute(only('randomMix'), 'separate', false)).toBe('/random/mix');
+    expect(resolveStartRoute(only('randomMix'), 'hub', false)).toBeNull();
+  });
+});