feat: dark theme applies to editor content area

Three changes make the editor respect the active theme:

1. useThemeManager now derives app-specific CSS variables (--bg-primary,
   --text-primary, --border-primary, etc.) from the theme's core colors,
   so the editor's EditorTheme.css variables resolve correctly for both
   light and dark themes.

2. BlockNoteView theme prop is now dynamic (isDark ? 'dark' : 'light')
   instead of hardcoded to 'light', so BlockNote's own UI chrome (menus,
   toolbars) matches the active theme.

3. Removed forced light-mode class removal from main.tsx that was
   preventing dark mode from being applied.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/SettingsPanel.test.tsx

@@ -40,6 +40,7 @@ const mockThemeManager: ThemeManager = {
   themes: [],
   activeThemeId: null,
   activeTheme: null,
+  isDark: false,
   switchTheme: vi.fn(),
   createTheme: vi.fn().mockResolvedValue('untitled'),
   reloadThemes: vi.fn(),