fix(shared): remove Canvas image fallback

[quanruzhuoxiu]

Summary

• Remove the browser Canvas image-processing fallback and standardize browser and worker processing on Photon.
• Keep Sharp as the Node.js backend, while allowing mixed browser/Node runtimes to fall back from Sharp to Photon.
• Remove obsolete async compatibility branches that only existed for the Canvas implementation.
• Isolate Photon module loading so browser and worker behavior can be tested deterministically.

Why

The previous implementation could produce a three-backend chain involving Sharp, Photon, and Canvas. Canvas also depends on DOM APIs, so it cannot provide a consistent fallback in worker environments. The image pipeline should have at most two native/WASM-backed implementations, with failures surfaced instead of silently switching to a third backend.

User impact

• Browser and worker environments now use Photon consistently.
• Photon initialization failures preserve their original cause.
• Pure Node.js environments preserve Sharp failures instead of replacing them with a misleading Photon availability error.

Validation

• pnpm --filter @midscene/shared test -- tests/unit-test/image/get-photon.test.ts tests/unit-test/image/index.test.ts tests/unit-test/transform.test.ts
• npx nx build @midscene/shared
• pnpm run lint
• Verified that no Canvas fallback implementation or runtime references remain in the shared image pipeline.