fix(web-ui): skip idle render pipeline and fix FSR vertical flip

[stackia]

Summary

• Skip the WebGL render pipeline entirely when both auto-deinterlace and picture enhancement are off, since it would only reproduce the raw video — falls back to the plain <video> element like an ineligible resolution
• Remove the now-redundant passthrough filter GPU pass by sampling the uploaded frame texture directly when no source stage runs
• Fix a vertical-flip regression this exposed: EASU/RCAS fragment shaders sample gl_FragCoord directly instead of the vertex shader's v_texCoord, so the existing u_flipY uniform never reached them and raw-texture presents rendered upside down — add a matching u_flipY uniform to both fragment shaders
• Relocate in-shader GLSL comments in fsr.ts to JSDoc above each shader constant (comments inside template literals ship raw into the production bundle)
• Add observability logging for FSR enablement and canvas resize events

Test plan

• pnpm run type-check:tsc / pnpm run lint:biome pass
• Verified in browser (devlab) across all four toggle combinations, checking image orientation and console logs:
  • auto-deinterlace off + enhancement off → raw <video> fallback, pipeline stopped
  • auto-deinterlace off + enhancement on → raw texture → FSR, correct orientation (previously upside down)
  • auto-deinterlace on + enhancement off → bwdif → passthrough, correct orientation
  • auto-deinterlace on + enhancement on → bwdif → FSR, correct orientation, no regression
• Confirmed built bundle contains no leaked shader comment text

[github-actions]

Documentation preview

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• Preview: https://thankful-water-0a297bf00-625.eastasia.1.azurestaticapps.net
• Commit: 3c8f708