Make dashboard work from proxied origins via same-origin proxy (#1111)

* Restore dashboard session proxy routes

Change-Id: I36ffc3727ce44100121bc94a81510a5f009ee0bc
Signed-off-by: Thomas Kosiewski <tk@coder.com>

* Port dashboard frontend and docs

Change-Id: I80356f64d618dab9d07b610ba67def14539f98ac
Signed-off-by: Thomas Kosiewski <tk@coder.com>

* docs: restore dashboard note in skill

Change-Id: Id0913c64e7a6f2cbbfc429ef03b34dae185d8487
Signed-off-by: Thomas Kosiewski <tk@coder.com>

* fix: tighten dashboard proxy same-origin checks

Change-Id: I792bc859a24cd47314bd46c94344ef3dfb7d6db5
Signed-off-by: Thomas Kosiewski <tk@coder.com>

---------

Signed-off-by: Thomas Kosiewski <tk@coder.com>

cli/src/output.rs

@@ -2554,6 +2554,11 @@ Running 'agent-browser dashboard' with no subcommand is equivalent to 'dashboard
 
 The dashboard runs as a standalone background process, independent of
 browser sessions. All sessions automatically stream to the dashboard.
+It works from http://localhost:4848 or a proxied/forwarded URL that
+reaches the dashboard server, such as https://dashboard.agent-browser.localhost
+or a Coder workspace URL. The browser stays on the dashboard origin;
+session tabs, status, and stream traffic are proxied internally, so
+session ports do not need to be exposed.
 
 Options:
   --port <n>           Port for the dashboard server (default: 4848)