[Web Engine] Support local screenshot and golden testing in felt test #191314

Part of #191313

Problem

In the Flutter Web Engine (`lib/web_ui`), running screenshot tests locally with `felt test` bypasses comparison entirely. `image_compare.dart` stubs out golden fetching and unconditionally returns `'OK'`, deferring all visual regression testing to remote CI tryjobs.

Solution

1. Fetch approved positive baselines from Skia Gold's REST API and cache them in `/.dart_tool/web_goldens_cache/baselines/`.
2. Implement local in-memory pixel diffing (both precise and fuzzy 3x3 neighborhood matching).
3. On comparison failure, save `actual.png`, `expected.png`, and `diff.png` (with highlighted differences in magenta) under `.dart_tool/web_goldens_cache/failures/<test_name>/`, and output clickable `file://` links.
4. Support `--offline` to run against cached baselines and `--refresh-goldens` to force re-downloading.

Critical Edge Cases & Implementation Gotchas

• Device Pixel Ratio (DPR) Alignment ([web] Flutter test suite in the web needs love. #143124): Headless Chrome starts with default DPR = 1.0 unless explicitly passed `--force-device-scale-factor=X`. Local diffing must enforce matching DPR flags when launching Chrome to avoid false-positive diff failures against Skia Gold CI baselines.

Related Issues & Historical Context

• [web] download goldens from https for windows web engine unit tests  #63710: [web] download goldens from https for windows web engine unit tests (Satisfied by REST baseline downloading; close on landing)
• [web] skipped image tests failing gold checks when enabled. #87933: [web] skipped image tests failing gold checks when enabled (Downstream validation target)
• [web] Flutter test suite in the web needs love. #143124: [web] Flutter test suite in the web needs love