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>

docs/web-tabs.md

@@ -147,6 +147,21 @@ layers cooperate so a dashboard talking to its own backend just works:
    using its `Referer` to identify the dashboard. This only fires for a request
    that already missed every Codeman route, and never for one that resolves to a
    real route, which is what keeps it from being an authentication bypass.
+5. The same script **masks the proxy prefix off the page's own URL** before any
+   of the page's code runs (`history.replaceState` to the path the page would see
+   on its own origin). A single-page app routes on `location.pathname` at boot,
+   and `/webview/<cap>/` is a path no app has a route for: without this, a React
+   Router / Vue Router / Next dev server painted its HTML and CSS and then replaced
+   them with its own "page not found" the moment its script ran. The page only
+   *reads* the masked path; every URL it emits still goes through the layers above.
+6. A navigation the page starts **itself** after that — `location.reload()` (a dev
+   server's full-reload HMR), a root-absolute `location.href = '/login'` — now
+   targets Codeman's root with no capability anywhere on it. Codeman recognises
+   that request by shape (a top-level `<iframe>` navigation asking for HTML, for a
+   path it does not serve) and answers a static page that does nothing but tell
+   the owning tab which path was lost; the tab remounts the frame inside the
+   prefix at that path. It never counts as a failed login, so a dev server that
+   reloads on every save cannot rate-limit its user out of Codeman.
 
 On top of that, the proxy answers those requests with CORS headers. That sounds
 wrong for same-host requests, but a sandboxed iframe has an *opaque* origin, so the
@@ -160,10 +175,12 @@ then every API call fails, which looks like the dashboard being broken.
   EventSource, normal markup, the DOM sinks a page uses to build markup at runtime,
   and `url()` inside stylesheets. Something that constructs requests by an unusual
   route can still slip through. Symptom: the page renders but a panel stays empty.
-- **Root-absolute `location` navigation.** A dashboard that navigates itself with
-  `location.href = '/login'` escapes the prefix, because `Location.href` is
-  unforgeable and cannot be patched the way the other sinks are. A relative
-  `location.href = 'login'` is fine (`<base>` covers it).
+- **Root-absolute `location` navigation is recovered, not prevented.** `Location`
+  is unforgeable, so `location.href = '/login'` or `location.reload()` really does
+  leave the prefix; the frame comes back through the recovery hop in layer 6 above,
+  which needs a browser that sends `Sec-Fetch-Dest` (every current one; iOS Safari
+  since 16.4). Older browsers show Codeman's 404 in the frame; the tab's **Reload**
+  button puts it back.
 - **Cross-origin redirects are not followed.** If a dashboard bounces to a different
   host (an external SSO provider, say), the proxy hands the redirect back unchanged
   rather than relaying it, because relaying would make this an open proxy. Use