feat(machine): operator branding — optional logo-dark.png variant

Sibling-file convention: drop a logo-dark.png alongside logo.png in
/var/lib/bitspire/branding/ and the renderer uses it whenever the
effective color mode is dark, falling back to logo.png when absent.
No branding.json change — the file name itself is the contract.

Wiring:
- electron/main.ts:loadBranding() reads logo-dark.png and base64-encodes
  it into logoDarkDataUrl on the IPC payload
- composables/useTheme.ts exposes an `isDark` computed that resolves
  the 'system' colorMode via the prefers-color-scheme media query (and
  reacts to OS-level dark-mode changes via the existing listener)
- composables/useBranding.ts switches logoUrl reactively based on isDark
- IdleView already binds to logoUrl — no template change needed

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/machine/electron/preload.ts

@@ -41,6 +41,7 @@ export interface BrandingConfig {
   customColors?: Record<string, string>
   customColorsDark?: Record<string, string>
   logoDataUrl: string | null
+  logoDarkDataUrl: string | null
 }
 
 /**