Fix UI issues: locale switching, dark theme, loading, edit support

- Fix i18n: remove $state rune (SSR incompatible in .ts files),
  use reactive localeVersion counter in layout to trigger re-render
  on locale change. Language switching now works immediately.
- Fix dark theme: add global CSS rules for input/select/textarea to
  use theme colors, override browser autofill in dark mode, set
  color-scheme for native controls (scrollbars, checkboxes)
- Collapsible sidebar: toggle button (▶/◀) with persistent state,
  icons-only mode when collapsed. Theme/language buttons moved to
  bottom above user info.
- Loading skeletons: all pages show animated pulse placeholders
  while data loads, eliminating content flicker on tab switch
- Edit support: Servers, Trackers, and Targets now have Edit buttons
  that open the form pre-filled with current values. Save calls PUT.
  Sensitive fields (API key, bot token) can be left empty to keep
  current value when editing.
- CLAUDE.md: add dev server restart rules

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -5,15 +5,17 @@
 	import { getAuth } from '$lib/auth.svelte';
 	import PageHeader from '$lib/components/PageHeader.svelte';
 	import Card from '$lib/components/Card.svelte';
+	import Loading from '$lib/components/Loading.svelte';
 
 	const auth = getAuth();
 	let users = $state<any[]>([]);
 	let showForm = $state(false);
 	let form = $state({ username: '', password: '', role: 'user' });
 	let error = $state('');
+	let loaded = $state(false);
 
 	onMount(load);
-	async function load() { try { users = await api('/users'); } catch {} }
+	async function load() { try { users = await api('/users'); } catch {} finally { loaded = true; } }
 
 	async function create(e: SubmitEvent) {
 		e.preventDefault(); error = '';
@@ -33,6 +35,8 @@
 	</button>
 </PageHeader>
 
+{#if !loaded}<Loading />{:else}
+
 {#if showForm}
 	<Card class="mb-6">
 		{#if error}<div class="bg-[var(--color-error-bg)] text-[var(--color-error-fg)] text-sm rounded-md p-3 mb-4">{error}</div>{/if}
@@ -72,3 +76,5 @@
 		</Card>
 	{/each}
 </div>
+
+{/if}