feat: clean navbar icons, floating theme bubble, auto theme on first visit

- Navbar: remove theme switcher, clean nav icons (no text labels),
  active state is accent dot underline + icon color — no pill bg
- useTheme: drop 'system' from stored type; first visit = auto
  by system preference, stays unset until user explicitly clicks
- ThemeBubble: fixed bottom-right pill with Sun/Moon icon + label,
  hover lift, click toggles light ↔ dark and persists to localStorage
- App.tsx: render ThemeBubble inside ThemeProvider

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

client/src/components/ThemeBubble.tsx

@@ -0,0 +1,31 @@
+import { Sun, Moon } from 'lucide-react';
+import { useTheme } from '@/hooks/useTheme';
+
+export default function ThemeBubble() {
+  const { resolvedTheme, setTheme } = useTheme();
+  const isDark = resolvedTheme === 'dark';
+
+  return (
+    <button
+      onClick={() => setTheme(isDark ? 'light' : 'dark')}
+      title={isDark ? 'Светлая тема' : 'Тёмная тема'}
+      aria-label="Переключить тему"
+      className={[
+        'fixed bottom-5 right-5 z-40',
+        'flex items-center gap-2 h-10 px-3.5 rounded-full',
+        'border border-[var(--border)]',
+        'bg-[var(--surface)] shadow-[0_4px_20px_rgba(0,0,0,0.18)]',
+        'text-[var(--fg-secondary)] text-xs font-semibold',
+        'transition-all duration-200 ease-out',
+        'hover:shadow-[0_6px_24px_rgba(0,0,0,0.25)] hover:-translate-y-0.5',
+        'active:scale-95 active:translate-y-0',
+        'select-none cursor-pointer backdrop-blur-sm',
+      ].join(' ')}
+    >
+      {isDark
+        ? <><Sun className="h-4 w-4 text-amber-400" strokeWidth={1.8} /><span className="hidden sm:inline">Светлая</span></>
+        : <><Moon className="h-4 w-4 text-indigo-400" strokeWidth={1.8} /><span className="hidden sm:inline">Тёмная</span></>
+      }
+    </button>
+  );
+}