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

README.md

@@ -1,4 +1,8 @@
-# 🦞 PinchChat
+<p align="center">
+  <img src="public/logo.png" alt="PinchChat" width="120" />
+</p>
+
+<h1 align="center">PinchChat</h1>
 
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