Add automated installer with OS detection and curl pipe support

- install.sh: One-command installer with macOS/Ubuntu/Arch detection
  - Supports curl pipe (remote) and local execution
  - Interactive sudo prompt for Linux, user-level by default
  - Idempotent rc file edits with marker blocks
  - Backs up existing configs before overwriting
  - Installs: fonts, tmux, starship, fetch scripts, iTerm2 colors
  - --uninstall flag restores backups and removes all changes
- Update .gitignore to include fonts/ directory
- Update README.md with install instructions
- Update TERMINAL-SETUP.md to reference automated installer

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

TERMINAL-SETUP.md

@@ -1,8 +1,11 @@
-# 🦊 AygeaNight Terminal Setup Guide
+# AygeaNight Terminal Setup Guide
 
 Tokyo Night base · Aygea brand accent colors
 Baby blue, sapphire, pink, silver — dark navy background
 
+> **The recommended way to install is the automated installer: `./install.sh`**
+> This guide covers the manual step-by-step process for reference or customization.
+
 ---
 
 ## Files in this repo