Add a local UI mock server

[longquanzheng]

Summary

• add a concurrency-safe Go in-memory mock API plus one-command Vite HMR through make mock
• add collapsible Mock Controls, visible start/approval loading states, polling pause and Retry, browser-refresh restoration, Reset, and one-shot failures
• cover the full OpenAPI response contract, mock control lifecycle, React behavior, and Playwright mock journey while keeping production free of mock routes
• bump the template contract to 1.2.0 and migrate embedded skills to the pinned dex-skills monorepo entries dex-app-builder and dex-sdk

Why

Product teams can approve frontend interactions before connecting to Dex, while make check still proves the real FDG 2.0, Worker replacement, Timer, RPC, and durability behavior.

Validation

• GOCACHE=/tmp/dex-template-go-cache DEXCLI=/tmp/dexcli-template-ci make check
• official quick_validate.py for both embedded skill wrappers
• manual visual QA of the mock UI in the local browser

Follow-up

After this PR is merged, dex-skills will pin the resulting main commit, require template 1.2.0 and the mock commands, update dex-app-builder guidance, set the plugin display name to Dex, and release manifests as 0.12.0.