fix(viewer): skip post-processing pipeline when WebGPU is unavailable

[b9llach]

Problem

On a browser where navigator.gpu is undefined (Safari without the WebGPU flag, most iOS WebKit builds, older Chrome on machines without a WebGPU device), the WebGPURenderer falls back to WebGL2. When that happens, the TSL-based post-processing pipeline in post-processing.tsx can't be built — RenderPipeline, ssgi, denoise, and the MRT pass setup from three/webgpu / three/tsl are all WebGPU-only.

The existing error path in useFrame retries the pipeline up to 3 times with a 500ms delay between attempts. Each retry fights the direct-render fallback, and the net result is "scene renders for about a second, then goes black and stays black."

Fix

Add a short-circuit at the top of the pipeline-setup useEffect:

const hasWebGPU =
  typeof navigator !== 'undefined' && typeof navigator.gpu !== 'undefined'
if (!hasWebGPU) {
  console.warn('[viewer] WebGPU unavailable — rendering without post-processing (SSGI, outlines, denoise).')
  hasPipelineErrorRef.current = true
  renderPipelineRef.current = null
  return
}

When the guard fires, hasPipelineErrorRef.current is set at setup time, so useFrame's existing if (hasPipelineErrorRef.current || !renderPipelineRef.current) branch takes the direct renderer.render(scene, camera) path and never attempts the TSL pipeline. The retry loop is skipped entirely.

Scope

• WebGPU mode: navigator.gpu is defined, the guard passes, nothing changes. SSGI, outlines, denoise all still run.
• No WebGPU: the guard fires, direct render only, no post FX (the scene is still visible — just flat shading).
• WebGPU API exposed but device creation fails at runtime: not handled specially — falls through the existing try { … } catch block exactly like today.

Pairs well with #233 (await renderer.init() in the gl factory), which fixes the direct-render path itself so the fallback actually works in WebGL2.

Test

On a WebGL2-only browser:

• Before: scene blinks on, retries loop, goes permanently black, console spam.
• After: one-time [viewer] WebGPU unavailable warning, scene renders without post FX, stays rendered.