feat: redesign login page with split-panel layout

[eugenioenko]

Summary

• Split-panel layout: gradient brand panel on the left, clean white form on the right
• Mobile responsive: brand collapses to a compact horizontal strip at the top
• Smooth CSS transitions on inputs (focus ring), primary button (shadow + press), secondary button (hover)
• Dark mode via prefers-color-scheme
• No new dependencies, no JS changes, all theming overrides still work

Layout

Desktop (≥ 800px)

┌─────────────────┬──────────────────────┐
│  [gradient]     │                      │
│                 │   Welcome back       │
│   [logo]        │   Sign in to ...     │
│   AppTitle      │                      │
│   tagline...    │   Username ________  │
│                 │   Password ________  │
│                 │                      │
│                 │   [ Sign in ]        │
└─────────────────┴──────────────────────┘

Mobile (< 800px)

┌──────────────────────────┐
│ [logo]  AppTitle         │  ← gradient strip
├──────────────────────────┤
│   Welcome back           │
│   Username ____________  │
│   Password ____________  │
│   [ Sign in ]            │
└──────────────────────────┘

Test plan

• Desktop: brand panel visible on left, form on right
• Mobile (<800px): brand strip at top, form below
• Dark mode: correct colors throughout
• Custom themeLogoUrl renders in brand panel
• Custom themeCssInline overrides still apply
• Passkey button and divider render correctly in password_and_passkey mode
• Error message styled correctly
• make test passes

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