refactor(admin): split admin page into tab sub-components

Split admin/+page.svelte (573 lines) into:
- UsersTab.svelte: user table with delete action
- TagsTab.svelte: tag list with inline rename and delete
- GroupsTab.svelte: groups table with inline edit + create form
- SystemTab.svelte: backfill buttons with own state

Page drops from 573 → ~40 lines.

Part of #75

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

frontend/src/routes/admin/UsersTab.svelte

@@ -0,0 +1,120 @@
+<script lang="ts">
+import { enhance } from '$app/forms';
+import { m } from '$lib/paraglide/messages.js';
+
+let {
+	users
+}: {
+	users: {
+		id: string;
+		username: string;
+		firstName?: string;
+		lastName?: string;
+		groups?: { id: string; name: string }[];
+	}[];
+} = $props();
+</script>
+
+<div class="overflow-hidden rounded-lg border border-line bg-surface shadow-sm">
+	<div class="flex items-center justify-between border-b border-line-2 p-6">
+		<h2 class="text-lg font-bold text-ink-2">{m.admin_section_users()}</h2>
+		<a
+			href="/admin/users/new"
+			class="inline-flex items-center gap-1 rounded-sm bg-primary px-4 py-2 font-sans text-xs font-bold tracking-widest text-white uppercase transition-opacity hover:opacity-80"
+		>
+			<svg class="h-4 w-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+				<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 4v16m8-8H4" />
+			</svg>
+			{m.admin_btn_new_user()}
+		</a>
+	</div>
+
+	<table class="min-w-full divide-y divide-line">
+		<thead class="bg-muted">
+			<tr>
+				<th class="px-6 py-3 text-left text-xs font-bold tracking-wider text-ink-2 uppercase"
+					>{m.admin_col_login()}</th
+				>
+				<th class="px-6 py-3 text-left text-xs font-bold tracking-wider text-ink-2 uppercase"
+					>{m.admin_col_full_name()}</th
+				>
+				<th class="px-6 py-3 text-left text-xs font-bold tracking-wider text-ink-2 uppercase"
+					>{m.admin_col_groups()}</th
+				>
+				<th class="px-6 py-3 text-right text-xs font-bold tracking-wider text-ink-2 uppercase"
+					>{m.admin_col_actions()}</th
+				>
+			</tr>
+		</thead>
+		<tbody class="divide-y divide-line bg-surface">
+			{#each users as user (user.id)}
+				<tr class="group/row hover:bg-muted">
+					<td class="px-6 py-4 text-sm font-medium whitespace-nowrap text-ink">
+						{user.username}
+					</td>
+					<td class="px-6 py-4 text-sm whitespace-nowrap text-ink-2">
+						{#if user.firstName || user.lastName}
+							{user.firstName ?? ''} {user.lastName ?? ''}
+						{:else}
+							<span class="text-ink-3 italic">–</span>
+						{/if}
+					</td>
+					<td class="px-6 py-4 text-sm text-ink-2">
+						<div class="flex flex-wrap gap-1">
+							{#if user.groups && user.groups.length > 0}
+								{#each user.groups as group (group.id)}
+									<span
+										class="rounded-full border border-blue-100 bg-blue-50 px-2 py-0.5 text-[10px] font-bold text-blue-700 uppercase"
+									>
+										{group.name}
+									</span>
+								{/each}
+							{:else}
+								<span class="text-xs text-ink-3 italic">{m.admin_no_groups()}</span>
+							{/if}
+						</div>
+					</td>
+					<td class="px-6 py-4 text-right whitespace-nowrap">
+						<div class="flex items-center justify-end gap-4">
+							<a
+								href="/admin/users/{user.id}"
+								class="text-sm font-bold tracking-wide text-accent uppercase hover:text-ink"
+							>
+								{m.btn_edit()}
+							</a>
+
+							<form
+								method="POST"
+								action="?/deleteUser"
+								use:enhance={({ cancel }) => {
+									if (!confirm(m.admin_user_delete_confirm({ username: user.username }))) {
+										cancel();
+									}
+									return async ({ update }) => {
+										await update();
+									};
+								}}
+								class="flex items-center"
+							>
+								<input type="hidden" name="id" value={user.id} />
+								<button
+									class="p-1 text-ink-3 transition-colors hover:text-red-600"
+									title={m.admin_btn_delete_user_title()}
+								>
+									<svg class="h-5 w-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+										<path
+											stroke-linecap="round"
+											stroke-linejoin="round"
+											stroke-width="2"
+											d="M19 7l-.867 12.142A2 2 0 0116.138 21H7.862a2 2 0 01-1.995-1.858L5 7m5 4v6m4-6v6m1-10V4a1 1 0 00-1-1h-4a1 1 0 00-1 1v3M4 7h16"
+										/>
+									</svg>
+								</button>
+							</form>
+						</div>
+					</td>
+				</tr>
+			{/each}
+		</tbody>
+	</table>
+</div>