refactor(machine): drop electron theme allowlist, defer to renderer

The VALID_THEMES set in electron/main.ts duplicated the renderer's
ThemeId list and silently coerced any unlisted branding.json theme to
null — which is how darkmatter regressed to the localStorage theme after
db074e2 added themes to the renderer but not this allowlist (fixed in
a0c2f38). Remove the second list entirely: pass raw.theme through and
let useTheme's applyBrandingTheme (themes[] + the 'custom' branch) be
the single validation point. Unknown values are ignored downstream, so
nothing reaches the DOM unvetted.

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

apps/machine/electron/main.ts

@@ -81,22 +81,6 @@ type BrandingConfig = {
   logoDarkDataUrl: string | null
 }
 
-const VALID_THEMES = new Set([
-  'gruvbox',
-  'catppuccin',
-  'cyberpunk',
-  'dracula',
-  'nord',
-  'tokyo-night',
-  'countrysidecastle',
-  'darkmatter',
-  'emeraldforest',
-  'lightgreen',
-  'neobrut',
-  'starrynight',
-  'custom',
-])
-
 function loadBranding(): BrandingConfig | null {
   const brandingDir = path.join(
     fs.existsSync('/var/lib/bitspire') ? '/var/lib/bitspire' : process.cwd(),
@@ -116,7 +100,10 @@ function loadBranding(): BrandingConfig | null {
     try {
       const raw = JSON.parse(fs.readFileSync(jsonPath, 'utf-8'))
       if (typeof raw.title === 'string') title = raw.title
-      if (typeof raw.theme === 'string' && VALID_THEMES.has(raw.theme)) theme = raw.theme
+      // No theme-name validation here: the renderer's `themes` list (plus its
+      // 'custom' branch) is the single source of truth. Pass the string through
+      // and let useTheme's applyBrandingTheme ignore anything it doesn't know.
+      if (typeof raw.theme === 'string') theme = raw.theme
       if (raw.custom_colors && typeof raw.custom_colors === 'object') {
         const { dark, ...flat } = raw.custom_colors as Record<string, unknown>
         const colors = Object.fromEntries(