Fix macOS/Linux sidecar build script and desktop setup docs

[adilzubair]

Summary

Fix the macOS/Linux sidecar build flow and update the desktop contributor setup docs to match the actual required steps.

This change fixes scripts/build-sidecar.sh so it produces the resource layout Tauri expects, uses repo-local cache directories during the sidecar build, and fails clearly if the expected sidecar binary is missing.

It also updates README.md and CONTRIBUTING.md so first-time contributors know they need to build the packaged sidecar resource before running npm run tauri dev.

Type of Change

• Scraper/source adapter
• Ranking/scoring
• Vector/profile intelligence
• Customizer/document generation
• Frontend/UI
• Backend/API/storage
• Docs/OSS hygiene
• CI/release
• Experimental automation

Testing

Commands run:

• npm run typecheck
• npm test
• npm run build
• backend/.venv/Scripts/python.exe -m pytest backend/tests
• cd src-tauri && cargo check
• Other:
  • bash scripts/build-sidecar.sh

Privacy And Safety

• No API keys, cookies, bearer tokens, resumes, local databases, or generated private documents are included.
• User-facing behavior is clear about what is core vs experimental.
• Ranking/filtering changes include explanations or tests where relevant.

Notes For Reviewers

This is a focused contributor-experience fix.

The main behavior change is in the macOS/Linux Bash sidecar helper, which now mirrors the working PowerShell flow more closely. The doc updates are intentionally small and only cover the missing step needed for full desktop setup.

[vercel]

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[adilzubair]

This overlaps with the existing PR that fixes the incorrect PyInstaller --distpath.

This PR also includes a few follow-up improvements:

• contributor docs for the missing sidecar build step
• repo-local cache env vars for the sidecar build
• a clearer failure check if the expected sidecar binary is not created

If maintainers prefer, I’m happy to narrow this PR to the non-overlapping follow-up changes only.