CE: Polish desktop app UI and tray UX

[ericflo]

What

Comprehensive UI polish pass on the desktop app's settings, dashboard, and model list.

Changes

• Design system: CSS custom properties for consistent colors, spacing, radii, and transitions throughout
• Dashboard: Redesigned status indicator with glow effect on connected state, stats displayed in rounded cards, error messages in a styled banner
• Models: Dedicated card with empty state (dashed border + icon), loading state with spinner during discovery, refresh button with spin animation, model tags with accent-colored pills and hover state
• Settings: Form organized into labeled sections (Connection, Identity, Performance, Behavior) with dividers; checkbox styled as a clickable card; input hover/focus states with box-shadow; save feedback uses checkmark icon
• Onboarding wizard: Panel slide-in animation, active step dot scale animation, inline validation (red glow on empty secret field instead of alert())
• Accessibility: focus-visible rings, keyboard-navigable tabs with role attributes, ARIA tab roles
• General: Smooth transitions on all interactive elements, consistent spacing, anti-aliased text rendering

Why

Desktop app should feel finished and polished, not prototype-quality.