feat: mobile-first responsive overhaul — safe areas, touch targets, bottom sheet, bottom nav

Viewport:
- Add `viewport` export with `viewport-fit=cover` (enables CSS safe-area-inset-*
  on bezel-less iOS/Android) and `interactive-widget=resizes-content` (prevents
  layout jumps from virtual keyboard open/close)

iOS zoom prevention:
- input.tsx, textarea.tsx: change text-sm (14px) → text-base sm:text-sm (16px)
  on mobile; iOS Safari auto-zooms any input with font-size < 16px

Touch targets:
- button.tsx: default h-10 → h-11 (44px), sm h-8 → h-9 (36px), icon h-10→h-11
  matching the 44×44px iOS HIG and WCAG 2.5.5 minimum
- Add touch-manipulation to buttons, inputs, textarea to eliminate 300ms tap delay

Dialog → mobile bottom sheet:
- On mobile (< 640px): anchored to bottom with full width, rounded top corners,
  slide-in-from-bottom / slide-out-to-bottom animation, drag handle affordance,
  max-h-[90svh] + overflow-y-auto to prevent viewport overflow
- Safe area padding: pb-[max(1.25rem,env(safe-area-inset-bottom))]
- On desktop (sm+): unchanged centered modal with zoom animation
- Close button gets a 32×32 tap area on mobile

Mobile navigation:
- New bottom-nav.tsx: persistent 3-tab bottom bar (مسیر/مرکز/لیگ) shown below
  md: breakpoint, hidden on desktop; 56px height + safe-area-inset-bottom;
  active tab gets heavier stroke and scale
- Only shown for authenticated non-admin users
- app-shell.tsx: injects BottomNav; main content gets dynamic padding-bottom to
  clear the bar (56px + safe area) on mobile, reverts to py-10 on md:
- Footer: safe-area-inset-bottom padding so content clears iPhone home indicator

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

frontend/src/components/app-shell.tsx

@@ -10,6 +10,7 @@ import { useAuth, useAuthHydrated } from "@/lib/auth-store";
 import { http } from "@/lib/api";
 import type { User } from "@/lib/types";
 import { hasInfiniteHearts } from "@/lib/hearts";
+import { BottomNav } from "@/components/bottom-nav";
 
 function SiteFooter() {
   return (
@@ -147,13 +148,21 @@ export function AppShell({ children }: { children: React.ReactNode }) {
           </button>
         </div>
       )}
-      <main className="mx-auto w-full max-w-6xl flex-1 px-4 py-10 sm:px-6">
+      <main
+        className="mx-auto w-full max-w-6xl flex-1 px-4 py-10 sm:px-6 md:pb-10"
+        // Bottom nav is 56px + safe area; only shown for authed non-admin on mobile
+        style={user && !pathname.startsWith("/admin") ? { paddingBottom: "calc(4.5rem + env(safe-area-inset-bottom, 0px))" } : undefined}
+      >
         {allowChildren ? children : null}
       </main>
-      <footer className="border-t border-border/70 bg-muted/20 backdrop-blur">
+      <footer
+        className="border-t border-border/70 bg-muted/20 backdrop-blur"
+        style={{ paddingBottom: "env(safe-area-inset-bottom, 0px)" }}
+      >
         <SiteFooter />
       </footer>
       {user && <ChatWidget />}
+      {user && !pathname.startsWith("/admin") && <BottomNav />}
     </div>
   );
 }