Add the appliance web UI look, interface names, MCP and REST API, local zone

Web UI: dark/light themes (appliance layout, sidebar, switch checkboxes),
theme preference on the Advanced page, Changes page with the pending diff.

Interface names: router.interfaces.<name>.mac, lans.<l>.deviceName and
pppoe.vlanName; a rename is staged for the next reboot by router-apply.

Remote access: an MCP server (POST /mcp) and an OpenAPI REST API
(/api/v1, /api/openapi.json) with the same tokens, scopes, limits and audit
log; tools to read status, settings, leases, logs (get_logs), and to edit
and check the saved settings (never applied). Tokens on the API access page.

Firewall: a reserved `local` zone (the router itself) as the destination of
rules, so the web UI or SSH can be allowed from the WAN.

PLAN.md lists what is still left.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

modules/router/pppoe.nix

@@ -12,7 +12,13 @@ let
   pppoe = wan.ipv4.pppoe;
 
   # Device carrying the PPPoE session: the VLAN on the uplink, or the uplink itself.
-  device = if pppoe.vlan == null then wan.interface else "${wan.interface}.${toString pppoe.vlan}";
+  device =
+    if pppoe.vlan == null then
+      wan.interface
+    else if pppoe.vlanName != null then
+      pppoe.vlanName
+    else
+      "${wan.interface}.${toString pppoe.vlan}";
 
   # Unit that appears once udev has seen the device (VLAN devices are created
   # by networkd after boot has begun, so pppd must not start before it).