Fix worker web UI navigation route fallback

[Xinyuexyyyyy]

Summary

• serve the Web UI entrypoint for extensionless browser navigation paths such as /canvas
• keep API/auth/debug/static asset paths from falling back to the Web UI
• apply the same navigation-path check to the Web UI login gate

Tests

• npm exec -- vitest run tests/unit/app/webUiAssets.spec.ts tests/contract/controlSurface/controlSurfaceHttpServer.webShell.spec.ts --reporter=dot
• npm exec -- tsc -b tsconfig.node.json tsconfig.web.json --noEmit
• npm exec -- prettier --check src/app/main/controlSurface/webUiAssets.ts src/app/main/controlSurface/http/webUiEntryGate.ts tests/unit/app/webUiAssets.spec.ts tests/contract/controlSurface/controlSurfaceHttpServer.webShell.spec.ts
• git diff --check

[CLAassistant]

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