fix(web): move the app origin off 6276, bound the publish route

Copilot review round 2 on #2100, plus a collision the review did not find.

The default app-origin port was 6276 — which is already
RUNNER_OAUTH_CALLBACK_DEFAULT_PORT, the CLI/TUI loopback OAuth callback.
That port is fixed precisely so an OAuth app can pre-register
http://127.0.0.1:6276/oauth/callback, which makes it the one listener here
that cannot move when something takes it first. A running --web would hold
it and break a later --cli/--tui OAuth flow at its registered redirect URI,
and docs/mcp-app-review.md has those two running side by side. This
listener has a documented dynamic fallback, so this listener yields: the
default is now 6278 (6277 skipped — v1's retired proxy port, which the
migration guide tells people to stop forwarding). A test asserts the two
defaults stay distinct.

Review comments:

- POST /api/app-document now applies hono's bodyLimit BEFORE parsing. The
  8MiB check ran after c.req.json() had buffered and parsed the whole
  request, and bounded only `html` — so bulk in `csp`, or in keys the route
  never reads, was fully materialized before the 413.
- `csp` is validated as an HTTP header value and length-bounded. The
  controller hands it to res.writeHead() verbatim, where Node throws
  ERR_INVALID_CHAR synchronously — inside the handler for a later,
  unrelated request, outside this route's error handling.
- The app-origin port now propagates everywhere the other two do: the CLI
  --print-handoff port-forward command (and its test), the Dockerfile's ENV
  and EXPOSE, and the remote-review SSH recipe in docs/mcp-app-review.md.
  Without it an app declaring _meta.ui.domain is unreachable through every
  documented remote workflow.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0177mPHAdECD18nLLCTwR5rh
Signed-off-by: cliffhall <cliff@futurescale.com>

Dockerfile

@@ -32,11 +32,14 @@ ENV HOST=0.0.0.0 \
     DANGEROUSLY_BIND_ALL_INTERFACES=true \
     CLIENT_PORT=6274 \
     MCP_SANDBOX_PORT=6275 \
+    MCP_APP_ORIGIN_PORT=6278 \
     MCP_AUTO_OPEN_ENABLED=false
-# 6275 is the MCP Apps sandbox, a second listener the browser reaches directly.
-# It is only needed for the Apps tab, so it is EXPOSEd but publishing it is
+# 6275 is the MCP Apps sandbox and 6278 the dedicated app origin (#2056) — two
+# further listeners the browser reaches DIRECTLY, so neither works through the
+# 6274 publish alone. Both are only needed for the Apps tab (6278 only for an
+# App declaring `_meta.ui.domain`), so they are EXPOSEd but publishing them is
 # optional — see the Docker section of the root README.
-EXPOSE 6274 6275
+EXPOSE 6274 6275 6278
 
 # Run as the non-root `node` user the base image ships. The inspector resolves
 # its runtime-state dir (default catalog, OAuth token storage) from `HOME`