chore: prepare desktop release verification

[ahamSel]

Summary

• Add a versioned browser tool schema layer and normalize tool declarations across Gemini, OpenAI-compatible, and Anthropic providers.
• Add deterministic browser smoke coverage, cross-platform desktop smoke handling, and unsigned desktop build helpers.
• Add Windows/Linux desktop package workflow support plus release/docs updates for unsigned artifacts and verification steps.

Validation

• npm run test
• npm run lint
• npm run browser:smoke
• npm run health
• npm run desktop:build:win
• npm run desktop:smoke
• Docker Linux verification: npm run test, npm run browser:smoke, npm run desktop:build:linux, xvfb-run -a npm run desktop:smoke

Notes

• Windows artifacts are unsigned and may show SmartScreen warnings.
• Linux packaging now produces AppImage plus .zip; the .tar.gz target was avoided because electron-builder used slow maximum compression for that path.