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)/layout.tsx

@@ -57,6 +57,9 @@ export default async function StudentLayout({ children }: { children: React.Reac
           <Link href="/wrapped" className="text-sm hover:underline" style={{ color: "var(--muted-foreground)" }}>
             Wrapped
           </Link>
+          <Link href="/tools" className="text-sm hover:underline" style={{ color: "var(--muted-foreground)" }}>
+            Инструменты
+          </Link>
           <Link href="/questions" className="text-sm hover:underline" style={{ color: "var(--muted-foreground)" }}>
             Вопросы
           </Link>