feat(web): support a reverse-proxy base URL (--base-url / CODEMAN_BASE_URL)

Codeman can now be mounted under a sub-path behind a reverse proxy that
forwards the prefix unchanged (e.g. https://host/codeman/). Default is `/`
(root), which is byte-identical to the historical behavior.

Design — few choke points, mirrored ingress/egress:
- src/config/base-path.ts: pure single-source normalize/validate/join/strip.
- Server ingress: stripBasePath() inside Fastify rewriteUrl, so routes stay
  declared prefix-agnostic; un-prefixed requests (hooks, health, docker bridge
  hitting the raw port) pass through unchanged.
- Server egress: one onSend hook prepends the base to root-absolute Location
  headers (covers all redirects).
- HTML: renderIndexHtml points <base href> at the mount and injects
  window.__CODEMAN_BASE__ — ONLY when a base is set (inert at root).
- Frontend runtime URLs: CodemanBase.url() route builder in constants.js,
  applied transparently by a fetch wrapper and explicitly at the
  EventSource/WebSocket/window.open/<img|iframe|a>-src sites.
- sw.js derives its base from self.location; manifest uses relative start_url/scope.
- Web-tab proxy: proxyPrefixFor(cap, basePath) is the single base-aware root that
  cascades to the injected <base>, HTML/attr rewrites, runtimeUrlShim, Set-Cookie
  Path and Location; capabilityFromReferer strips the base off the browser Referer,
  while the ingress parsers stay base-agnostic (rewriteUrl already stripped it).

--base-url rides the daemon relaunch (buildWebArgs) and the service unit
(resolveServicePlan). constants.js is guarded against a missing `window` for
isolated unit-test contexts.

Tests: test/base-path.test.ts (pure helpers), base-path coverage in
webview-proxy/render-index-html/daemon-control; CodemanBase stubbed in the
vm-isolated panels-ui test contexts. Docs: Remote-Access.md (sub-path section +
nginx example), security-architecture.md env table, CLAUDE.md pattern.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XUkPBxbumnct6qSrx4JDju

src/daemon-control.ts

@@ -45,6 +45,8 @@ export interface WebLaunchOptions {
   host: string;
   port: number;
   https: boolean;
+  /** Reverse-proxy sub-path prefix (normalized: '' for root, or '/foo'). */
+  basePath?: string;
   titleHostname?: string;
   allowUnauthenticatedNetwork?: boolean;
   multiuser?: boolean;
@@ -87,6 +89,7 @@ export interface DaemonStatus {
 export function buildWebArgs(options: WebLaunchOptions): string[] {
   const args = ['web', '--host', options.host, '--port', String(options.port)];
   if (options.https) args.push('--https');
+  if (options.basePath) args.push('--base-url', options.basePath);
   if (options.titleHostname) args.push('--title-hostname', options.titleHostname);
   if (options.allowUnauthenticatedNetwork) args.push('--allow-unauthenticated-network');
   if (options.multiuser) args.push('--multiuser');