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/app/layout.tsx

@@ -1,4 +1,4 @@
-import type { Metadata } from "next";
+import type { Metadata, Viewport } from "next";
 import localFont from "next/font/local";
 import "./globals.css";
 import { Providers } from "@/components/providers";
@@ -37,6 +37,13 @@ export const metadata: Metadata = {
   description: "مسیر یادگیری واقعی پرگار",
 };
 
+export const viewport: Viewport = {
+  width: "device-width",
+  initialScale: 1,
+  viewportFit: "cover",
+  interactiveWidget: "resizes-content",
+};
+
 export default function RootLayout({ children }: { children: React.ReactNode }) {
   return (
     <html