feat: interactive runtime config UI + dynamic vacations and staffing

[Vhivi]

Summary

This PR introduces a full interactive configuration workflow and makes planning generation use runtime config updates immediately, without backend restart.

What changed

Backend

• Added runtime config endpoints:
  • GET /config
  • GET /config/default
  • PUT /config
• Added config validation using JSON Schema before saving.
• Added atomic config write and in-memory hot reload after save.
• Generation now uses active runtime config directly.
• Added clearer error handling for invalid runtime config values.

Solver

• Added support for dynamic vacation names (no longer hard-coded to Jour/Nuit/CDP only).
• Added staffing_requirements support for simultaneous agent count per vacation/day.
• Added fallback staffing value (1) when not configured.
• Updated constraints/objective internals to work safely when some vacations are absent (e.g. no Nuit, no CDP).

Frontend

• Added interactive configuration mode in App.vue:
  • Load active config
  • Load default config
  • Edit vacations, durations, staffing, agents, holidays, solver values
  • Save to config.json
• Added auto-save-before-generate behavior.
• Fixed generation flow so planning still runs immediately after autosave.

Documentation

• Updated README.md, docs/config-reference.md, CHANGELOG.md for:
  • runtime config API
  • dynamic vacations
  • staffing requirements
  • interactive config workflow

Tests

• Backend: python -m pytest -q -> passing
• Frontend: npm test -- --runInBand -> passing
• Frontend lint: npm run lint -> passing
• Frontend build: npm run build -> passing

Notes

• backend/config.json is local runtime config; tests were validated with config.example.json compatibility.
• Infeasible staffing/vacation combinations correctly return No solution found..