feat(dashboard): Members tab on the app detail page

A new "Members" tab is rendered between Domains and Settings for
callers whose `my_role` on the app is `app_admin` (owners always;
explicit member-app_admins; admins do not see it — they're only
implicit editors and can't manage memberships).

The tab lets the caller:

- See every explicit member of the app with username, email, instance-
  role chip, app-role chip, and joined date. Inactive users render
  greyed-out so admins know the row exists.
- Pick a `member`-instance user from a dropdown and grant viewer /
  editor / app_admin access. The dropdown is populated from
  `/admin/admins` filtered to active members not already on the app.
- Promote / demote / remove existing members via the shared
  `ActionMenu` kebab. Removal goes through `ConfirmModal`.

Member-with-app_admin callers see a disabled add form with an
explanatory message — they have authority to manage memberships but
can't browse the user directory (gated on `InstanceManageUsers`),
which is a known phase-3.5 caveat to revisit in a follow-up.

Also extends `RoleChip` with an `appRole` prop and palette for app
roles, and adds an `appMembers` namespace to api.ts mirroring the
`domains` shape.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dashboard/src/lib/RoleChip.svelte

@@ -1,15 +1,24 @@
 <script lang="ts">
 	import type { InstanceRole } from '$lib/auth';
+	import type { AppRole } from '$lib/api';
 
 	interface Props {
-		role: InstanceRole;
+		role?: InstanceRole;
+		appRole?: AppRole;
 		size?: 'sm' | 'md';
 	}
 
-	let { role, size = 'md' }: Props = $props();
+	let { role, appRole, size = 'md' }: Props = $props();
+
+	// Display label: app roles read better with a space ("app admin")
+	// than their wire form ("app_admin").
+	const label = $derived(
+		appRole ? appRole.replace('_', ' ') : (role ?? '')
+	);
+	const cls = $derived(appRole ? `chip-${appRole}` : `chip-${role}`);
 </script>
 
-<span class="chip chip-{role}" class:sm={size === 'sm'}>{role}</span>
+<span class="chip {cls}" class:sm={size === 'sm'}>{label}</span>
 
 <style>
 	.chip {
@@ -42,4 +51,19 @@
 		color: #cbd5e1;
 		border-color: #334155;
 	}
+	.chip-app_admin {
+		background: #4c1d95;
+		color: #c4b5fd;
+		border-color: #6d28d9;
+	}
+	.chip-editor {
+		background: #1e3a8a;
+		color: #93c5fd;
+		border-color: #1d4ed8;
+	}
+	.chip-viewer {
+		background: #1f2937;
+		color: #9ca3af;
+		border-color: #374151;
+	}
 </style>