Add typed settings areas to the REST API

/api/v1/settings/router/firewall, .../router/lans/<name>, .../system, down to
a single option: GET reads, PUT replaces, PATCH merges, DELETE removes, with
the settings' ETag/If-Match and the same limits as the whole-document edit
(editSettings is now shared). Values are checked against a JSON Schema made
from the option documentation (schema.go reads the types, which nixosOptionsDoc
describes in words); a wrong type, a value out of range or an unknown key is
refused with its place. The OpenAPI document has the area paths with their
schemas; the API reference page lists every area's options.

Tests: unit tests for the type reader, the validator and the paths; the real
option file and a router's own settings validate (ROUTER_OPTIONS_JSON,
LIVE_SETTINGS_JSON); a subtest in apply.

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

pkgs/router-webui/apiref.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"net/http"
 	"slices"
+	"sort"
 	"strings"
 )
 
@@ -69,5 +70,48 @@ func apiReference(host string) []apiDoc {
 func (a *app) apiReference(w http.ResponseWriter, r *http.Request, sess session) {
 	a.render(w, "apiref", page{Title: "API reference", CSRF: sess.csrf, LoggedIn: true, Data: map[string]any{
 		"Base": "https://" + r.Host, "Ops": apiReference(r.Host), "HasOptions": a.optionsFile != "",
+		"Areas": settingsAreas(a.optionsFile), "Host": r.Host,
 	}})
 }
+
+type areaOption struct{ Name, Type, Default, Description string }
+
+type areaDoc struct {
+	Path    string
+	Options []areaOption
+}
+
+// settingsAreas lists the areas below router/ with their options (from the
+// option documentation): what a person can read and write there.
+func settingsAreas(optionsFile string) []areaDoc {
+	if optionsFile == "" {
+		return nil
+	}
+	entries, err := loadOptions(optionsFile)
+	if err != nil {
+		return nil
+	}
+	byArea := map[string][]areaOption{}
+	for _, e := range entries {
+		if len(e.Loc) < 3 || e.Loc[0] != "router" {
+			continue
+		}
+		desc := strings.TrimSpace(e.Description)
+		if i := strings.Index(desc, "\n\n"); i > 0 {
+			desc = desc[:i]
+		}
+		desc = strings.Join(strings.Fields(desc), " ")
+		if len(desc) > 300 {
+			desc = desc[:300] + "…"
+		}
+		name := strings.Join(e.Loc[2:], ".")
+		byArea[e.Loc[1]] = append(byArea[e.Loc[1]], areaOption{name, e.Type, literal(e.Default), desc})
+	}
+	var areas []areaDoc
+	for k, opts := range byArea {
+		sort.Slice(opts, func(i, j int) bool { return opts[i].Name < opts[j].Name })
+		areas = append(areas, areaDoc{Path: "/api/v1/settings/router/" + k, Options: opts})
+	}
+	sort.Slice(areas, func(i, j int) bool { return areas[i].Path < areas[j].Path })
+	return areas
+}