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/lib/components/Loading.svelte

@@ -0,0 +1,9 @@
+<script lang="ts">
+	let { lines = 3 } = $props<{ lines?: number }>();
+</script>
+
+<div class="space-y-3 animate-pulse">
+	{#each Array(lines) as _}
+		<div class="bg-[var(--color-muted)] rounded-lg h-16"></div>
+	{/each}
+</div>

frontend/src/lib/i18n/index.ts

@@ -1,6 +1,6 @@
 /**
- * Simple i18n store using Svelte 5 runes.
- * Supports nested keys like "nav.dashboard".
+ * Simple i18n module. Uses plain variable (no $state rune)
+ * so it works in both SSR and client contexts.
  */
 
 import en from './en.json';
@@ -10,7 +10,7 @@ export type Locale = 'en' | 'ru';
 
 const translations: Record<Locale, Record<string, any>> = { en, ru };
 
-let currentLocale = $state<Locale>('en');
+let currentLocale: Locale = 'en';
 
 export function getLocale(): Locale {
 	return currentLocale;