fix: reset Electron backend health state after process failures

[ubugeeei]

Finding

Electron caches backendHealthyPromise globally and never clears it when the backend exits or a health check times out. A failed startup can poison later render attempts in the same app session.

Evidence

• electron/main.ts waitForHealthz returns cached backendHealthyPromise.
• backendProcess.on('exit') only clears backendProcess, not backendHealthyPromise.
• The timeout rejection path leaves the rejected promise cached.

Acceptance criteria

• Clear backendHealthyPromise on backend exit/error and after health timeout/failure.
• Preserve successful health caching while the backend process remains alive.
• npm run build:electron passes.

[ubugeeei]

対応済みです。修正は PR #1 / commit 9cf4498 に反映済みです。