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/main.ts

@@ -60,16 +60,20 @@ function loadEnvFile() {
 
 loadEnvFile()
 
-// Branding loader — reads /var/lib/bitspire/branding/{logo.png,branding.json}
-// and surfaces them on get-config. Per issue #47: local-file is the V1
-// source; V2 will overlay a higher-priority Nostr-event source from
-// satmachineadmin (issue #48). Renderer applies via useBranding().
+// Branding loader — reads /var/lib/bitspire/branding/{logo.png,
+// logo-dark.png,branding.json} and surfaces them on get-config. Per
+// issue #47: local-file is the V1 source; V2 will overlay a
+// higher-priority Nostr-event source from satmachineadmin (issue #48).
+// Renderer applies via useBranding().
 type BrandingConfig = {
   title: string | null
   theme: string | null
   customColors?: Record<string, string>
   customColorsDark?: Record<string, string>
   logoDataUrl: string | null
+  /** Optional dark-mode logo variant. Renderer falls back to logoDataUrl
+   *  when null. */
+  logoDarkDataUrl: string | null
 }
 
 const VALID_THEMES = new Set([
@@ -94,6 +98,7 @@ function loadBranding(): BrandingConfig | null {
   let customColors: Record<string, string> | undefined
   let customColorsDark: Record<string, string> | undefined
   let logoDataUrl: string | null = null
+  let logoDarkDataUrl: string | null = null
 
   const jsonPath = path.join(brandingDir, 'branding.json')
   if (fs.existsSync(jsonPath)) {
@@ -131,16 +136,27 @@ function loadBranding(): BrandingConfig | null {
     }
   }
 
+  const logoDarkPath = path.join(brandingDir, 'logo-dark.png')
+  if (fs.existsSync(logoDarkPath)) {
+    try {
+      const buf = fs.readFileSync(logoDarkPath)
+      logoDarkDataUrl = `data:image/png;base64,${buf.toString('base64')}`
+    } catch (e) {
+      console.warn('[Electron] Failed to read logo-dark.png:', e)
+    }
+  }
+
   if (
     title === null &&
     theme === null &&
     !customColors &&
     !customColorsDark &&
-    logoDataUrl === null
+    logoDataUrl === null &&
+    logoDarkDataUrl === null
   ) {
     return null
   }
-  return { title, theme, customColors, customColorsDark, logoDataUrl }
+  return { title, theme, customColors, customColorsDark, logoDataUrl, logoDarkDataUrl }
 }
 
 // Determine if we're in development

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
 }
 
 /**