feat: integrate PinchChat logo throughout the app

- Use logo.png as favicon (replaces emoji SVG)
- Show logo in header next to title
- Show larger logo on login screen
- Add OG image meta tag
- Add centered logo in README
- Closes feedback item #8

src/components/Header.tsx

@@ -1,4 +1,4 @@
-import { Menu, Bot, Sparkles, LogOut } from 'lucide-react';
+import { Menu, Sparkles, LogOut } from 'lucide-react';
 import type { ConnectionStatus, Session } from '../types';
 import { useT } from '../hooks/useLocale';
 import { LanguageSelector } from './LanguageSelector';
@@ -22,9 +22,7 @@ export function Header({ status, sessionKey, onToggleSidebar, activeSessionData,
         <Menu size={20} />
       </button>
       <div className="flex items-center gap-3 flex-1 min-w-0">
-        <div className="flex h-9 w-9 items-center justify-center rounded-2xl border border-white/8 bg-zinc-800/40">
-          <Bot className="h-4 w-4 text-cyan-200" />
-        </div>
+        <img src="/logo.png" alt="PinchChat" className="h-9 w-9 rounded-2xl" />
         <div className="min-w-0">
           <div className="flex items-center gap-2">
             <span className="font-semibold text-zinc-300 text-sm tracking-wide">{t('header.title')}</span>

src/components/LoginScreen.tsx

@@ -1,5 +1,5 @@
 import { useState, useEffect } from 'react';
-import { Bot, Sparkles, Eye, EyeOff, Loader2 } from 'lucide-react';
+import { Sparkles, Eye, EyeOff, Loader2 } from 'lucide-react';
 import { useT } from '../hooks/useLocale';
 
 interface Props {
@@ -54,9 +54,7 @@ export function LoginScreen({ onConnect, error, isConnecting }: Props) {
       <div className="w-full max-w-md mx-4">
         {/* Logo */}
         <div className="flex flex-col items-center gap-3 mb-8">
-          <div className="flex h-14 w-14 items-center justify-center rounded-2xl border border-white/10 bg-zinc-800/60 shadow-lg shadow-cyan-500/5">
-            <Bot className="h-7 w-7 text-cyan-200" />
-          </div>
+          <img src="/logo.png" alt="PinchChat" className="h-20 w-20 drop-shadow-lg" />
           <div className="flex items-center gap-2">
             <h1 className="text-2xl font-bold text-zinc-200 tracking-wide">{t('login.title')}</h1>
             <Sparkles className="h-5 w-5 text-cyan-300/60" />