refactor: unify project structure, improve security, and enhance deployment

- Fix WireGuard private key leak in API responses and config updates
- Update systemd service to serve from repo root with adjusted sandbox
- Add CLI flags, idempotency, and dev mode to install.sh
- Extract common utilities to lib/common.py and webui/api/common.py
- Migrate frontend to htmx for simpler, more maintainable UI
- Update docs to reflect current architecture and deployment model
- Vendor htmx dependencies per project requirements

webui/templates/zones.html

@@ -62,7 +62,7 @@
 <div class="modal-overlay" id="create-zone-modal" onclick="if(event.target===this) closeModal('create-zone-modal')">
     <div class="modal">
         <h2>Create Zone</h2>
-        <form hx-post="/api/firewall/zones" hx-swap="none" hx-on::after-request="if(evt.detail.successful){ closeModal('create-zone-modal'); refreshTable('/api/firewall/zones', document.getElementById('zone-grid'), renderZones); showSuccessToast('Zone created'); }">
+        <form hx-post="/api/firewall/zones" hx-encoding="json" hx-swap="none" hx-on::after-request="if(evt.detail.successful){ closeModal('create-zone-modal'); refreshTable('/api/firewall/zones', document.getElementById('zone-grid'), renderZones); showSuccessToast('Zone created'); }">
             <div class="form-group">
                 <label for="zone-name">Zone Name</label>
                 <input type="text" id="zone-name" name="name" placeholder="e.g., trusted, dmz, external" required>