feat: sessions management, web manifest, square icon-only buttons, remove lang select

- Backend: migration 002 adds user_agent/ip/last_used_at to refresh_tokens
- Backend: GET /api/auth/sessions + DELETE /api/auth/sessions/{id} endpoints
- Frontend: profile page — sessions section (browser, IP, datetime, revoke)
- Frontend: web manifest + SVG icon for PWA support
- Frontend: remove language selector from all navbars (moved to profile page)
- Frontend: neu-btn--icon-sm class for square icon-only buttons (theme/logout/edit)
- Frontend: manifest link added to all 9 HTML pages

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/css/neu.css

@@ -215,9 +215,8 @@ input, select, textarea { font-family: inherit; font-size: inherit; }
   border-radius: var(--neu-radius-md);
 }
 
-/* Bouton icône carré taille sm — se déclenche automatiquement si le bouton
-   ne contient qu'un <i> (ex: theme, logout, edit mode) */
-.neu-btn--sm:has(> i:only-child) {
+/* Bouton icône carré (taille sm) */
+.neu-btn.neu-btn--icon-sm {
   width: 2rem;
   height: 2rem;
   padding: 0;