Redesign login/auth UX - Enable Sync flow

[tomymaritano]

Problem

Login is buried in Settings > Account. No onboarding for new users. Magic link auth assumes prior registration knowledge.

Proposed Flow

1. App works 100% locally (no account required)
2. After N notes created, show subtle "Sync across devices" prompt
3. "Enable Sync" button in sidebar footer or status bar
4. Modal with clear value proposition + email input
5. Magic link → deep link verification → auto-sync starts
6. Success toast + persistent sync indicator

Files to Change

• renderer/components/sync/LoginModal.tsx — redesign as "Enable Sync" modal
• renderer/stores/authStore.ts — add isFirstTimeUser detection
• renderer/components/Sidebar.tsx or StatusBar.tsx — add entry point
• web/app/(marketing)/auth/verify/ — improve fallback UX when deep link fails

Acceptance Criteria

• New user sees sync prompt after creating 3+ notes
• "Enable Sync" accessible from sidebar/status bar
• Modal explains value proposition before asking for email
• Resend magic link option with countdown timer
• Success state with sync indicator visible