feat: rebuild desktop UI to match Figma + website, hero slider improvements

- Desktop sidebar (262px, blue gradient, white pill nav), topbar (search + bell + avatar), responsive shell rewritten
- Desktop homepage: immersive hero with Ken Burns animation, pill category chips, date badge cards matching mvnew.eventifyplus.com/home
- Desktop calendar: 60/40 two-column layout with white background
- Desktop profile: full-width banner + 3-column event grids
- Desktop learn more: hero image + about/venue columns + gallery strip
- Desktop settings/contribute: polished to match design system
- Mobile hero slider: RepaintBoundary, animated dots with 44px tap targets, 5s auto-scroll, 8s post-swipe delay, shimmer loading, dynamic event type badge, human-readable dates
- Guest access: requiresAuth false on read endpoints
- Location fix: show place names instead of lat/lng coordinates
- Version 1.6.1+17

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

lib/core/constants.dart

@@ -1,10 +1,16 @@
 import 'package:flutter/material.dart';
 
 class AppConstants {
-  // Layout
+  // Layout — breakpoints
   static const double desktopBreakpoint = 820;
+  static const double wideDesktopBreakpoint = 1200;
   static const double tabletBreakpoint = 600;
 
+  // Desktop sidebar
+  static const double sidebarExpandedWidth = 262;
+  static const double topBarHeight = 64;
+  static const double desktopHorizontalPadding = 24;
+
   // Padding & Radius
   static const double defaultPadding = 16;
   static const double cardRadius = 14;