Remove drop-in config and use absolute paths in service file

Eliminate drop-in dependency by having install.sh replace
with absolute paths in the service file. EnvironmentFile loads vars from
.env directly.

Changes:
- Remove email_forwarder.service.d/ directory
- Update install.sh to sed service file for absolute paths
- Simplify README to rely on install.sh

install.sh

@@ -53,12 +53,8 @@ setup_systemd() {
     # Copy service files
     cp *.service *.timer "$HOME/.config/systemd/user/"
 
-    # Copy and configure drop-in
-    mkdir -p "$HOME/.config/systemd/user/email_forwarder.service.d"
-    cp email_forwarder.service.d/override.conf.sample "$HOME/.config/systemd/user/email_forwarder.service.d/override.conf"
-
-    # Replace placeholders
-    sed -i "s|/path/to/your/email_forwarder|$PROJECT_DIR|g" "$HOME/.config/systemd/user/email_forwarder.service.d/override.conf"
+    # Replace placeholders in service file
+    sed -i "s|\${PROJECT_DIR}|$PROJECT_DIR|g" "$HOME/.config/systemd/user/email_forwarder.service"
 
     # Reload systemd
     systemctl --user daemon-reload