feat: Add comprehensive address-first geolocation system for events

This implementation provides automatic geocoding and map integration:

- **Event Model Enhancements:**
  - Automatic geocoding callback using OpenStreetMap Nominatim API
  - 3-tier fallback system: exact coordinates → city-based → country default
  - Fallback coordinates for major French cities (Paris, Lyon, Marseille, etc.)
  - Robust error handling that prevents event creation failures

- **User-Friendly Event Forms:**
  - Address-first approach - users just enter addresses
  - Hidden coordinate fields (auto-generated behind scenes)
  - Real-time geocoding with 1.5s debounce
  - "Ma position" button for current location with reverse geocoding
  - "Prévisualiser" button to show map links
  - Smart feedback system (loading, success, warnings, errors)

- **Enhanced Event Show Page:**
  - Map provider links (OpenStreetMap, Google Maps, Apple Plans)
  - Warning badges when approximate coordinates are used
  - Address-based URLs for better map integration

- **Comprehensive JavaScript Controller:**
  - Debounced auto-geocoding to minimize API calls
  - Multiple geocoding strategies (manual vs automatic)
  - Promise-based geolocation with proper error handling
  - Dynamic map link generation with address + coordinates

- **Failure Handling:**
  - Events never fail to save due to missing coordinates
  - Fallback to city-based coordinates when exact geocoding fails
  - User-friendly warnings when approximate locations are used
  - Maintains existing coordinates on update failures

🤖 Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

app/javascript/controllers/index.js

@@ -21,3 +21,6 @@ application.register("header", HeaderController);
 
 import QrCodeController from "./qr_code_controller";
 application.register("qr-code", QrCodeController);
+
+import EventFormController from "./event_form_controller";
+application.register("event-form", EventFormController);