feat: add pi theme JSON system with theme picker

[Kabochar]

Summary

Add a theme system that loads pi CLI theme JSON files from ~/.pi/agent/themes/ and applies them to the whole UI: a top-bar theme picker lists every installed theme set (e.g. gruvbox, solarized, each with dark/light variants).

Changes

Core

• lib/theme.ts: parser/resolver for pi CLI theme JSON — vars references, 256-color indices, brightness-based dark/light inference, base-dark.json/base-light.json set pairing, 51-token -> CSS variable mapping
• lib/prism-theme.ts: syntax highlighting bound to theme CSS variables, so code colors follow the active theme without re-render lag
• app/api/themes routes: theme set discovery + per-variant resolution
• hooks/useTheme.ts: themeName state, setTheme, CSS variable application with localStorage persistence and old-value (light/dark) migration
• app/layout.tsx: bootstrap pre-sets data-theme to avoid flash of unstyled content

UI

• Top-bar theme picker panel reusing the existing top-panel mechanism

Web adaptation (mapping layer only, theme JSON files untouched)

• WCAG contrast guarantees: text/muted >= 4.5:1, dim >= 3:1, accent family >= 4.5:1, border >= 3:1 (hue preserved, lightness adjusted only)
• Sidebar surface color (--bg-sidebar), user bubble differentiation, accent-tinted selected state
• --composer-focus-bg derived from theme surface
• Hardcoded blue accents (message bubble border, explorer dot, image border) replaced with theme variables

Tests

• lib/theme.test.mjs: 9 node:test cases — set aggregation, variant pairing, single-file fallback, contrast guarantees across all four variants

Compatibility

• Default theme (no selection) is unchanged: globals.css fallbacks cover all new variables
• Theme JSON format is the pi CLI standard, so existing terminal themes work out of the box

Verification

• node --test lib/theme.test.mjs (9/9 pass)
• tsc --noEmit, npm run lint clean
• Manually verified theme switching, syntax highlighting and focus state across light/dark