Add Obsidian Toolbox — 4 syntax generators under /tools

Inside-LMS toolbox for registered students: callout-CSS, YAML-frontmatter,
theme CSS-variables, Style Settings generators. Auth inherited from (student)
route group; no public surface, no email gate.

- Pure generators in src/lib/tools/* with Vitest (33 tests)
- Shared YAML-safe scalar quoter (_shared/yaml.ts) used by frontmatter +
  style-settings: unquoted user input was silently breaking YAML (tags '#x'
  -> null, color default '#7C3AED' -> null, 'a: b' -> parse error)
- hex validation in callout (no NaN), date input constrained, clipboard +
  analytics calls guarded
- Prisma ToolUsage model + migration; logToolUsage Server Action (auth-first,
  10-min dedup window per user/tool)
- Tool index /tools + ToolCard (explicit lucide map, no import *) + header link

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/(student)/tools/frontmatter/page.tsx

@@ -0,0 +1,15 @@
+import { FrontmatterForm } from "./FrontmatterForm";
+
+export const metadata = { title: "Генератор YAML-frontmatter — Obsidian Toolbox" };
+
+export default function FrontmatterToolPage() {
+  return (
+    <main className="mx-auto w-full max-w-5xl px-6 py-8">
+      <h1 className="text-2xl font-bold tracking-wide" style={{ color: "var(--foreground)" }}>Генератор YAML-frontmatter</h1>
+      <p className="mt-1 text-sm" style={{ color: "var(--muted-foreground)" }}>Шаблон свойств заметки: имя, тип, значение по умолчанию.</p>
+      <div className="mt-6 card-aubade p-4">
+        <FrontmatterForm />
+      </div>
+    </main>
+  );
+}