Fix systemd service to use python3 directly instead of uv

Changed service to run python3 directly instead of 'make start'
to avoid dependency on uv being installed system-wide.

Service file changes:
- ExecStart: /usr/bin/python3 rotary_phone_web.py (instead of make)
- User/Group: berwn (match actual user, not hardcoded pi)
- WorkingDirectory: /home/berwn/wedding-phone
- PATH includes user's .local/bin for any user-installed packages

Install script improvements:
- Automatically substitutes correct username
- Substitutes correct home directory in PATH
- Substitutes correct working directory
- More robust sed replacements with proper escaping

This fixes the error:
  make: uv: No such file or directory
  make: *** [Makefile:8: start] Error 127

Now the service works regardless of whether uv is installed,
as it uses system python3 which is always available.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

wedding-phone.service

@@ -5,11 +5,11 @@ Wants=network.target sound.target
 
 [Service]
 Type=simple
-User=pi
-Group=pi
-WorkingDirectory=/home/pi/wedding-phone
-Environment="PATH=/home/pi/.local/bin:/usr/local/bin:/usr/bin:/bin"
-ExecStart=/usr/bin/make start
+User=berwn
+Group=berwn
+WorkingDirectory=/home/berwn/wedding-phone
+Environment="PATH=/home/berwn/.local/bin:/usr/local/bin:/usr/bin:/bin"
+ExecStart=/usr/bin/python3 rotary_phone_web.py
 Restart=always
 RestartSec=10