Add social sharing buttons and QR code for events

Public event page:
- Share buttons: X (Twitter), Facebook, Instagram (copy link), TikTok (copy link), copy link
- Buttons use url_encode for share URLs with event title + URL
- Instagram/TikTok copy to clipboard (no direct share URL support)
- Consistent brutal design with aria-labels

Organizer dashboard:
- Share X, Facebook, copy link buttons per event in events list
- QR code download button per event
- Route /mon-compte/evenement/{id}/qrcode: generates 400px PNG QR code via Endroid
- QR code points to public event URL, downloaded as qrcode-{slug}.png

JS module:
- assets/modules/share.js: initShare() handles data-share-copy buttons
- Copies URL to clipboard, shows checkmark for 1.5s then restores icon
- 4 tests (no buttons, copy, checkmark restore, multiple buttons)

Social icons already displayed via _social_icons.html.twig component

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

assets/modules/share.js

@@ -0,0 +1,12 @@
+export function initShare() {
+    document.querySelectorAll('[data-share-copy]').forEach(btn => {
+        btn.addEventListener('click', () => {
+            const url = btn.dataset.shareCopy
+            globalThis.navigator.clipboard.writeText(url).then(() => {
+                const original = btn.innerHTML
+                btn.innerHTML = '<svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M5 13l4 4L19 7"/></svg>'
+                setTimeout(() => { btn.innerHTML = original }, 1500)
+            })
+        })
+    })
+}