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

test/render-index-html.test.ts

@@ -228,3 +228,42 @@ describe('WebServer.renderIndexHtml', () => {
     expect(html).not.toContain('gesture-codeman.js');
   });
 });
+
+describe('WebServer.renderIndexHtml reverse-proxy base path', () => {
+  const BASE_TEMPLATE = ['<head>', '<base href="/">', '<title>Codeman</title>', '</head>', '<body></body>'].join('\n');
+
+  function makeBaseServer(basePath: string) {
+    // constructor: (port, https, testMode, host, titleHostname, allowUnauth, basePath)
+    const server = new WebServer(0, false, true, '127.0.0.1', undefined, false, basePath);
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (server as any).indexHtmlTemplate = BASE_TEMPLATE;
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (server as any).readSettings = vi.fn(async () => ({}));
+    return server;
+  }
+
+  it('is inert at root — base tag unchanged and no base global injected', async () => {
+    const server = makeBaseServer('');
+    const html = await render(server);
+    expect(html).toContain('<base href="/">');
+    // At root the frontend reads a MISSING __CODEMAN_BASE__ as root, so nothing is
+    // injected and the historical output is byte-identical.
+    expect(html).not.toContain('__CODEMAN_BASE__');
+  });
+
+  it('points the base tag and the base global at a sub-path mount', async () => {
+    const server = makeBaseServer('/codeman');
+    const html = await render(server);
+    expect(html).toContain('<base href="/codeman/">');
+    expect(html).toContain('window.__CODEMAN_BASE__="/codeman"');
+    // The global rides right after <base>, before any (deferred) script.
+    expect(html.indexOf('window.__CODEMAN_BASE__')).toBeLessThan(html.indexOf('</head>'));
+  });
+
+  it('normalizes a raw operator prefix passed to the constructor', async () => {
+    const server = makeBaseServer('codeman/');
+    const html = await render(server);
+    expect(html).toContain('<base href="/codeman/">');
+    expect(html).toContain('window.__CODEMAN_BASE__="/codeman"');
+  });
+});