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

pkgs/router-webui/templates/changes.html

@@ -0,0 +1,15 @@
+{{define "content"}}
+{{with .Data}}
+{{if .Diff}}
+<p>The saved settings differ from the confirmed (running) ones: <code>-</code> lines go away, <code>+</code> lines are new.
+   Nothing is applied until you press Apply.</p>
+<pre class="log">{{.Diff}}</pre>
+<div class="actions">
+  <form method="post" action="/apply"><input type="hidden" name="csrf" value="{{$.CSRF}}"><button>Apply</button></form>
+  <form method="post" action="/discard"><input type="hidden" name="csrf" value="{{$.CSRF}}"><button class="secondary">Discard changes</button></form>
+</div>
+{{else}}
+<p>No changes: the saved settings are the confirmed ones.</p>
+{{end}}
+{{end}}
+{{end}}