feat(machine): add markdown-driven support pages

Add support/help pages accessible via a ? button on the idle screen.
Pages are driven by .md files in /var/lib/lamassu-atm/support/ —
operators can customize content without rebuilding the app.

Features:
- Tabbed view with markdown rendering (via marked)
- Standalone URLs auto-render as QR codes (scannable from phone)
- Table URLs: click-to-reveal QR codes (prevents accidental scans)
- Yes/No rendered as green checkmarks / red X marks
- Wallet comparison table with download QR codes
- FAQ with Lightning-only clarification
- Support page with operator Nostr QR placeholder
- Custodial vs non-custodial footnote
- Large text for touchscreen accessibility
- Centered layout for short-content pages

Closes #36

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/machine/electron/preload.ts

@@ -79,6 +79,10 @@ contextBridge.exposeInMainWorld('electronAPI', {
   remediateTransaction: (txid: string, remediatedByTxid: string): Promise<boolean> =>
     ipcRenderer.invoke('state:remediate-transaction', txid, remediatedByTxid),
 
+  // Support pages
+  getSupportPages: (): Promise<{ id: string; title: string; content: string }[]> =>
+    ipcRenderer.invoke('support:get-pages'),
+
   // HAL hardware (runs in main process, exposed via IPC)
   halInit: (config: any): Promise<{ success: boolean; error?: string }> =>
     ipcRenderer.invoke('hal:init', config),
@@ -152,6 +156,7 @@ declare global {
       }) => Promise<void>
       emptyCashbox: () => Promise<void>
       remediateTransaction: (txid: string, remediatedByTxid: string) => Promise<boolean>
+      getSupportPages: () => Promise<{ id: string; title: string; content: string }[]>
       // HAL hardware IPC
       halInit: (config: any) => Promise<{ success: boolean; error?: string }>
       halDispense: (amounts: any) => Promise<any>