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>

backend/main.go

@@ -130,6 +130,8 @@ func main() {
 		r.Post("/api/auth/logout", authHandler.Logout)
 		r.Get("/api/auth/me", authHandler.Me)
 		r.Patch("/api/auth/preferences", authHandler.UpdatePreferences)
+		r.Get("/api/auth/sessions", authHandler.GetSessions)
+		r.Delete("/api/auth/sessions/{id}", authHandler.RevokeSession)
 
 		// Proxmox
 		r.Get("/api/proxmox/resources", proxmoxHandler.GetResources)