fix(dashboard): F-U-018 allow owners to invite owners directly

The invite-user modal offered only admin/member radios. Owner could
only be granted by editing an existing user — asymmetric with
editRoleOptions which lets owners assign owner directly. The original
intention was preventing the first-owner footgun but the asymmetry
was confusing.

Show the Owner radio only when me?.instance_role === 'owner'. The
help text ("Owners can't be created here — promote via Edit") still
shows for non-owner admins so the friction is preserved where it
matters.

AUDIT.md anchor: F-U-018.

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

dashboard/src/routes/users/+page.svelte

@@ -50,7 +50,10 @@
 
 	// Invite (create) modal --------------------------------------------------
 	let inviteOpen = $state(false);
-	let inviteForm = $state<{ username: string; email: string; instance_role: 'admin' | 'member' }>({
+	// F-U-018: Owner is selectable for the bootstrap case (owners can
+	// always create more owners; the asymmetry with editRoleOptions
+	// was unnecessary friction).
+	let inviteForm = $state<{ username: string; email: string; instance_role: InstanceRole }>({
 		username: '',
 		email: '',
 		instance_role: 'admin'
@@ -449,6 +452,12 @@
 			</label>
 			<fieldset class="field">
 				<legend>Role</legend>
+				{#if me?.instance_role === 'owner'}
+					<label class="radio">
+						<input type="radio" bind:group={inviteForm.instance_role} value="owner" />
+						<span>Owner — full instance control (irrevocable except by another owner).</span>
+					</label>
+				{/if}
 				<label class="radio">
 					<input type="radio" bind:group={inviteForm.instance_role} value="admin" />
 					<span>Admin — can manage users, scripts, and all apps.</span>
@@ -457,9 +466,9 @@
 					<input type="radio" bind:group={inviteForm.instance_role} value="member" />
 					<span>Member — only sees apps they're added to.</span>
 				</label>
-				<small>
-					Owners can't be created here — promote via Edit after creation.
-				</small>
+				{#if me?.instance_role !== 'owner'}
+					<small>Owners can't be created here — promote via Edit after creation.</small>
+				{/if}
 			</fieldset>
 			{#if inviteError}
 				<div class="error">{inviteError}</div>