feat(branding): per-brand default theme + palette via brand.json

Lets a deployer set the in-app color scheme a fresh visitor sees (e.g.
cfaun → darkmatter light) without forking. Two optional brand.json
fields, `theme` (light|dark|system) and `palette` (one of PALETTES),
distinct from `themeColor` which is PWA chrome only.

- vite-branding.ts surfaces them as VITE_BRAND_THEME / VITE_BRAND_PALETTE
  at module load, so the default applies app-wide (hub + all standalones)
  with no per-config wiring.
- theme-provider reads them as the INITIAL value of theme/palette; a
  user's stored choice in localStorage still wins and persists.
- Splits the catppuccin = bare `:root` invariant (now BASE_PALETTE, used
  by applyPalette to drop data-theme) from the configurable default.
  Without this, a non-catppuccin brand default would strip the
  data-theme attribute and silently render catppuccin instead.

Unset → the app's built-ins (dark + catppuccin), unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

vite-branding.ts

@@ -28,12 +28,35 @@ export interface Brand {
    * config keeps its hardcoded value.
    */
   backgroundColor?: string
+  /**
+   * Optional default in-app theme mode (light / dark / system). Sets the
+   * INITIAL value the theme-provider uses when the user has no saved
+   * preference; a user's later choice still wins and persists. Unset →
+   * the app's built-in default ('dark'). Distinct from `themeColor`,
+   * which is PWA chrome only.
+   */
+  theme?: 'light' | 'dark' | 'system'
+  /**
+   * Optional default in-app color palette (e.g. 'darkmatter'). Same
+   * initial-default semantics as `theme`. Must be one of the palettes in
+   * src/components/theme-provider (PALETTES). Unset → 'catppuccin'.
+   */
+  palette?: string
 }
 
 export const brand: Brand = JSON.parse(
   readFileSync(resolve(BRAND_DIR, 'brand.json'), 'utf-8'),
 )
 
+// Surface the brand's in-app theme defaults to the client as VITE_*
+// env vars (read by the theme-provider). Set here at module load — every
+// vite.<app>.config.ts imports this file — so the default applies
+// app-wide (hub + all standalones) without per-config wiring. Always
+// assigned (empty when unset) so a prior brand's value can't leak into a
+// later build in the same process.
+process.env.VITE_BRAND_THEME = brand.theme ?? ''
+process.env.VITE_BRAND_PALETTE = brand.palette ?? ''
+
 /**
  * Spread into a vite config's `resolve.alias` map. Lets components
  * import deployer-provided assets via `@brand/<file>` instead of