Point the panel link at the installation, not at the browser's origin

Both links out of the dashboard editor were built root-relative, so a portal
serving the app under `/i/{id}` got a URL to itself: the hub has no route
there and answers a bare 404. That is what a device link and "open what a
wall panel sees" both landed on.

They want different answers. The view link is for the person already looking,
so it takes the router's basepath — `appPath` in `lib/portal` is the same
prefix the router applies to every `Link`, for the places that step outside
it. The device link is for a screen, which cannot go through the portal at
all: the shell is served only to a portal session, and the credential that
page carries is the portal's rather than the panel's. So the server now says
where it answers, and `FRONTEND_HOST` is that answer — the same setting the
password-reset links already use.

Also fixes the panel branch in the query error handler, which compared a raw
pathname and so never fired under a portal.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AHpLJHozysQXjsxAyU1WHj

frontend/src/client/schemas.gen.ts

@@ -2054,6 +2054,32 @@ export const PanelsConfigSchema = {
     description: 'Every panel this installation knows about.'
 } as const;
 
+export const PanelsPublicSchema = {
+    properties: {
+        panels: {
+            items: {
+                '$ref': '#/components/schemas/PanelDef'
+            },
+            type: 'array',
+            title: 'Panels'
+        },
+        frontend_host: {
+            type: 'string',
+            title: 'Frontend Host',
+            default: ''
+        }
+    },
+    type: 'object',
+    title: 'PanelsPublic',
+    description: `The panels, and the address a device should be pointed at.
+
+The address is the server's own, because the browser's origin is not a
+reliable answer to it: an admin working through the portal is on the
+portal's origin, and a screen cannot be sent there — the portal serves a
+page only to someone holding a portal session, and the credential it hands
+that page is the portal's rather than the panel's.`
+} as const;
+
 export const PlacementSchema = {
     properties: {
         x: {