fix(webview): let a proxied single-page app route on its own path, and recover a frame that reloads

A dashboard served through a web tab saw `/webview/<cap>/` as its
`location.pathname`, and no app has a route for that: a React Router, Vue
Router or Vite dev-server page painted its HTML and CSS and then replaced
them with its own "page not found" the moment its script ran (reproduced
with a minimal history-routed page).

The proxy's runtime shim now rewrites the history entry to the path the
page would see on its own origin, before any page script runs. The base
element still resolves relative URLs inside the prefix and every root-
absolute sink is rewritten back into it, so only what the page READS
changes. With the document URL masked the Referer-keyed 404 rescue can no
longer help a request the shim misses, so the remaining URL-taking entry
points (`Worker`, `SharedWorker`, `navigator.sendBeacon`, `window.open`)
are covered by the shim as well.

A navigation the page starts itself afterwards — `location.reload()`
(a dev server's full-reload HMR), a root-absolute `location.href` — lands
on Codeman's root with no capability anywhere: no prefix in the path, no
cookie in an opaque-origin frame, a Referer naming the masked page. It is
recognised by shape (a top-level iframe navigation asking for HTML, for a
path Codeman does not serve) and answered with a static page whose only
script posts `{type:'codeman:webview-lost', path}` to the parent; the tab
that owns the frame (matched by `event.source`, never by the payload)
remounts it inside the prefix at that path, bounded per frame. The
unauthenticated form is answered in the auth middleware before the
credential checks, so a dev server that reloads on every save cannot
rate-limit its own user out of Codeman; the authenticated form (Basic
auth, trusted mode) is answered by the 404 handler.

Verified end to end against a history-routed page: boots on `/`, its
API call succeeds, a reload inside the frame comes back routed on the
path it had pushed, `location.href = '/about'` comes back on `/about`,
and a deep link opens on its path.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/web/server.ts

@@ -181,6 +181,7 @@ import {
   registerTabLayoutRoutes,
   tryWebviewRefererFallback,
 } from './routes/index.js';
+import { isLostWebviewFrameNavigation, lostWebviewFramePage, LOST_FRAME_PAGE_CSP } from './webview-proxy.js';
 import { CronService } from '../cron/cron-service.js';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
@@ -976,6 +977,15 @@ export class WebServer extends EventEmitter {
       // and the relay declines unless the Referer carries a live capability, so
       // genuinely unknown `/api` paths still get the envelope below.
       if (await tryWebviewRefererFallback(req, reply, this.basePath)) return reply;
+      // An authenticated web-tab frame (Basic auth, or trusted mode with a cookie)
+      // that navigated itself off its proxy prefix: the runtime shim masks the
+      // prefix so the page's router sees its own path, and a reload of that page
+      // lands here. The unauthenticated form is answered in the auth middleware.
+      if (!req.url.startsWith('/api') && isLostWebviewFrameNavigation(req)) {
+        reply.header('content-security-policy', LOST_FRAME_PAGE_CSP);
+        reply.header('cache-control', 'no-store');
+        return reply.type('text/html; charset=utf-8').send(lostWebviewFramePage());
+      }
       if (req.url.startsWith('/api')) {
         return reply.code(404).send(createErrorResponse(ApiErrorCode.NOT_FOUND, notFound));
       }