feat(admin): ask the panel to restart, from verdande's own settings

A container cannot replace its own image from the inside, so "update" is really
"ask whoever owns the container to recreate it" — Yggdrasil pulls `:latest` on the
way, which is what makes a restart an update at all. This removes the second
browser tab, not the panel's authority: the panel still does the work and still
records who asked.

Three settings, needed together, and the page names whichever are missing — an
operator reading "not configured" should not have to go and find out which of
three. The token is never sent to a browser; the page is told only whether it is
set.

Answers 202 rather than waiting. The panel stops this container as part of
answering, so the reply may never arrive, and a request cut off mid-flight is the
successful case wearing a failure's clothes. The interface believes neither answer
and polls /healthz until it responds.

Administrators only, and sessions only. A leaked API token that could restart the
server is a denial of service in one request.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

rune/verdande.yaml

@@ -78,6 +78,24 @@ gameskill:
       type: bool
       default: false
 
+    # Restarting from verdande's own settings, which is also how it picks up a new
+    # version: a container cannot replace its own image, so the panel recreates it
+    # and pulls :latest on the way. All three are needed together; the page names
+    # whichever are missing.
+    - key: VERDANDE_PANEL_URL
+      name: "Panel address, so verdande can ask to be restarted from its own settings"
+      type: string
+      default: ""
+    - key: VERDANDE_PANEL_TOKEN
+      name: "Panel API token — from an account with control of this server"
+      type: string
+      secret: true
+      default: ""
+    - key: VERDANDE_PANEL_SERVER_ID
+      name: "This server's id in the panel (the one in its URL)"
+      type: string
+      default: ""
+
   install:
     # The image has no shell, so the install step borrows one. It only has to make
     # the directory layout exist; verdande creates and migrates its own database on