fix: wait for server before running agents

[omribz156]

Description

Fixes #47

The Agents page now checks /api/health on load and keeps agent Run buttons disabled until the server responds. This avoids sending /api/run/:id requests while the Express server is still starting behind the Vite proxy.

Also adds localized button text for the startup state, plus README and changelog notes for the user-visible behavior.

This was implemented with Codex assistance, with the final patch manually reviewed and kept focused.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Verification

• npm ci --ignore-scripts
• npm run ui:build
• npx prettier --check ui/src/components/AgentCard.tsx ui/src/pages/AgentsPage.tsx ui/src/locales/en.json ui/src/locales/he.json
• npx eslint ui/src/components/AgentCard.tsx ui/src/pages/AgentsPage.tsx
• git diff --check
• README.md and CHANGELOG.md updated for the docs gate

npx tsc --project ui/tsconfig.json --noEmit still reports existing unrelated errors in ui/src/components/AppShell.tsx and ui/src/pages/PlaywrightDashboard.tsx, matching the current baseline seen on nearby Open-QA work.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes