Handle file watcher resource exhaustion

[galjos]

Summary

• clean up native watcher startup failures
• fall back to polling so the GUI and live updates remain available
• show polling mode in the GUI status and document it

Tests

• 204 tests with 100% coverage
• 9 GUI/TUI end-to-end tests
• real polling update after a forced EMFILE failure

Closes #89

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fc0c5e3) to head (44b1bab).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #90   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        11           
  Lines          502       502           
=========================================
  Hits           502       502           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.