test: Add CI smoke tests for Docker, Electron, and Web

[icebear0828]

This commit addresses #133, #134, and #135 by implementing independent CI smoke tests for each of the core components of the Codex Proxy application.

1. Docker: A workflow builds the image and checks its default and custom port behavior through port mapping + mounted config overlays, verifying the healthcheck status and endpoint response.
2. Electron: Playwright dependencies were added to the electron workspace, along with a smoke.test.ts that relies on xvfb and an unpacked application to assert basic window-launch behavior. A CI workflow handles the full build, prepare, unpack, and launch sequence.
3. Web: A new vitest file in src/__tests__/ validates Tailwind's CSS compilation (checking for color-scheme properties from dark mode plugin classes) and ensures the built Hono Node server can host the index.html payload appropriately on an arbitrary port.

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PR created automatically by Jules for task 9084205728708085973 started by @icebear0828

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