feat(ui): WebGL ambient background with day/night crossfade

[patschmittdev]

Summary

Adds a full-viewport WebGL ambient background to the app shell. It crossfades between a deep-forest night palette and a near-white day palette whenever the theme flips, and gently breathes the genesis aurora while an agent is streaming. The render loop runs only during the ~450ms theme morph and the activity breathe, then parks, so an idle window costs nothing. When WebGL is unavailable it falls back to a static CSS .app-ambient gradient, and it honors prefers-reduced-motion.

Changes

• AmbientCanvas (AmbientCanvas.tsx): owns the GL context, the theme observer (watches the .dark class), and a parked rAF loop; caps device-pixel-ratio at 1.5; disposes safely under React StrictMode.
• ambientScene (ambientScene.ts): pure shader + transition math (THEME_MS, vertex/fragment shaders, easeInOut, themeTarget), unit-tested in jsdom.
• AppShell: mounts the canvas behind a z-10 content layer and drives the breathe from streamingByMind / a2aStreamingByMind.
• index.css: .app-ambient static fallback gradient (self-contained oklch, no token dependencies).
• env.d.ts: adds a vite/client reference so the import.meta.hot HMR block type-checks under the root tsconfig.
• tsconfig.base.json: module: commonjs -> esnext. Required for import.meta to be legal under tsc --noEmit, and commonjs already conflicted with the moduleResolution: bundler set in the same file. tsc is --noEmit only here (Vite/esbuild own all emit), so this affects type-checking semantics, not output.

Test evidence

• npm run lint: clean (tsc + eslint + dependency-cruiser 515 modules / 0 violations + yaml + markdown).
• npm run typecheck: clean (exit 0).
• npm test: 2001 passed / 1 failed. The single failure is MindProfileService > rejects symlinked profile files, which fails at fs.symlinkSync(...) with EPERM: operation not permitted. That is a Windows symlink-privilege limitation (requires Developer Mode), not a test assertion. The file is untouched by this branch and the test passes in CI on Linux.

Notes

• No linked issue (feature initiated directly).
• No SDK, server, or packaging surface is touched, so those smoke tests were not required.