Add a separate 2D / 3D View switch

[nekomario28]

Summary

Implements the renderer hierarchy decided after #281: Three.js is not a thirteenth 2D Style. Instead, the viewer exposes a separate top-level View: 2D / 3D control, while each renderer keeps its own Style space.

Behavior

• 2D viewer keeps the existing twelve Style options unchanged.
• Adds a View control with 2D active and 3D Lab as the experimental switch.
• Three.js exposes the same View control with 3D Lab active and 2D as the return path.
• Three.js exposes its current renderer-local Style as Cosmic; this is intentionally separate from the twelve 2D styles and leaves room for future 3D styles.
• Routes remain /u/ and /three/; no premature route unification.
• Existing renderer-neutral state transfer remains authoritative for username/search/status/motion/activity/focus/depth/quality.
• Renderer navigation additionally carries style2d, so switching Galaxy Hybrid 2D → Cosmic 3D → 2D restores Galaxy Hybrid rather than resetting the 2D style.
• style2d is removed again when returning to 2D; Three.js render= is never carried into 2D.
• Dedicated 2D routes such as /radial/ receive the same View axis.
• Three.js remains visibly marked Lab/experimental.

Architecture

This deliberately models:

View
├─ 2D
│  └─ Style: existing 12 presets
└─ 3D (Lab)
   └─ Style: Cosmic (currently one option)

rather than mixing threejs into the existing Style enum.

Gates

• build-time injection is idempotent;
• generated JS is syntax checked and linted;
• 2D test asserts exactly twelve Style options and no Three.js/Cosmic entry;
• browser E2E verifies transferable state plus style2d restoration across 2D → 3D → 2D;
• dedicated /radial/ route verifies the same architecture;
• existing full Verify, 12-preset comparison, Chromium/WebKit and Three.js performance regressions remain enabled.

Refs #276.