Fix release image validation and prepare v0.2.0

[biancarosa]

The current dashboard removed the HTTP Proxy Dashboard heading, but both Docker validations still require it. Current main therefore compiles successfully and then fails its image smoke test, blocking releases and Railway deployment.

Validate the actual embedded dashboard JavaScript and its configured prefix instead of UI copy. The Dockerfile now fetches that asset, matching the existing runtime smoke script; both still reject the fallback page and require a healthy same-origin API.

Also correct the existing failing browser error test: force an actual local upstream disconnect, assert HTTP 502 and the sanitized body omission marker, and avoid public DNS and obsolete raw-error text expectations. Production proxy and UI behavior are unchanged. Includes the generated v0.2.0 changelog for the features already on main.

Validation:

• Reproduced the original Docker build failure locally after successful frontend and Go compilation.
• Fixed root-path image build and runtime smoke passed (non-root UID, dashboard, JS asset and API health).
• make check passed (Go unit/race coverage, E2E, lint, build).
• Existing-Chrome browser suite: all 7 passed, including the real failed-proxy exchange.
• OpenClaw-prefix image build and runtime smoke passed.
• Dashboard unit tests: 10 passed.
• Exact-head branch E2E workflow passed; final CI status is required before merge/tag.

This is release preparation for v0.2.0. Merging main can trigger the already-configured Railway deployment. It does not change k3s, networking, authentication, infrastructure, or the separate GitOps PR #31.