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/static/style.css

@@ -120,6 +120,8 @@ table tr:nth-child(even) td { background: var(--zebra); }
 pre.log { background: var(--code); padding: .85rem 1rem; border-radius: var(--radius); overflow-x: auto; font-size: 12.5px; max-height: 30rem; }
 code { font-size: .88em; overflow-wrap: anywhere; background: var(--code); padding: .1em .35em; border-radius: 3px; }
 pre code { background: none; padding: 0; }
+.opts td:first-child code { white-space: nowrap; overflow-wrap: normal; }
+.opts td { vertical-align: top; }
 .muted { color: var(--muted); }
 .bad { color: var(--bad); }
 .ok { color: var(--good); }