feat(frontend): invite-based registration UI

- Add /register route with invite code prefill, password show/hide
- Add /login?registered=1 success banner
- Add /admin/invites page: list, create, revoke, copy link
- Add Einladungen nav section to admin sidebar (ADMIN_USER perm)
- Add invite error codes to errors.ts
- Add 48 i18n keys across de/en/es
- Update hooks.server.ts to allow public access to invite/register API

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/admin/EntityNav.svelte

@@ -9,6 +9,7 @@ let {
 	userCount,
 	groupCount,
 	tagCount,
+	inviteCount,
 	canManageUsers,
 	canManageTags,
 	canManagePermissions,
@@ -17,6 +18,7 @@ let {
 	userCount: number;
 	groupCount: number;
 	tagCount: number;
+	inviteCount: number;
 	canManageUsers: boolean;
 	canManageTags: boolean;
 	canManagePermissions: boolean;
@@ -86,6 +88,23 @@ function handleKeydown(event: KeyboardEvent) {
 	</svg>
 {/snippet}
 
+{#snippet invitesIcon()}
+	<svg
+		class="h-5 w-5 flex-shrink-0 {isActive('invites') ? 'text-brand-mint' : 'text-white/40'}"
+		fill="none"
+		viewBox="0 0 24 24"
+		stroke="currentColor"
+		stroke-width="1.5"
+		aria-hidden="true"
+	>
+		<path
+			stroke-linecap="round"
+			stroke-linejoin="round"
+			d="M21.75 6.75v10.5a2.25 2.25 0 01-2.25 2.25h-15a2.25 2.25 0 01-2.25-2.25V6.75m19.5 0A2.25 2.25 0 0019.5 4.5h-15a2.25 2.25 0 00-2.25 2.25m19.5 0v.243a2.25 2.25 0 01-1.07 1.916l-7.5 4.615a2.25 2.25 0 01-2.36 0L3.32 8.91a2.25 2.25 0 01-1.07-1.916V6.75"
+		/>
+	</svg>
+{/snippet}
+
 {#snippet tagsIcon()}
 	<svg
 		class="h-5 w-5 flex-shrink-0 {isActive('tags') ? 'text-brand-mint' : 'text-white/40'}"
@@ -180,6 +199,18 @@ function handleKeydown(event: KeyboardEvent) {
 		/>
 	{/if}
 
+	{#if canManageUsers}
+		<EntityNavSection
+			variant="sidebar"
+			href="/admin/invites"
+			label={m.admin_tab_invites()}
+			isActive={isActive('invites')}
+			count={inviteCount}
+			onTabletTrigger={openFlyout}
+			icon={invitesIcon}
+		/>
+	{/if}
+
 	{#if canManageTags}
 		<EntityNavSection
 			variant="sidebar"
@@ -264,6 +295,18 @@ function handleKeydown(event: KeyboardEvent) {
 			/>
 		{/if}
 
+		{#if canManageUsers}
+			<EntityNavSection
+				variant="flyout"
+				href="/admin/invites"
+				label={m.admin_tab_invites()}
+				isActive={isActive('invites')}
+				count={inviteCount}
+				onFlyoutClick={closeFlyout}
+				icon={invitesIcon}
+			/>
+		{/if}
+
 		{#if canManageTags}
 			<EntityNavSection
 				variant="flyout"