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/lib/tools/__tests__/theme.test.ts

@@ -0,0 +1,19 @@
+import { describe, it, expect } from "vitest";
+import { buildThemeCss } from "@/lib/tools/theme";
+
+describe("buildThemeCss", () => {
+  const input = { background: "#0F0F0F", text: "#EAEAEA", accent: "#7C3AED", link: "#4EA1FF", border: "#333333" };
+  it("оборачивает в body { }", () => {
+    const css = buildThemeCss(input);
+    expect(css.startsWith("body {")).toBe(true);
+    expect(css.trim().endsWith("}")).toBe(true);
+  });
+  it("мапит accent в --interactive-accent", () => {
+    expect(buildThemeCss(input)).toContain("--interactive-accent: #7C3AED;");
+  });
+  it("содержит все 5 переменных", () => {
+    const css = buildThemeCss(input);
+    ["--background-primary", "--text-normal", "--interactive-accent", "--link-color", "--background-modifier-border"]
+      .forEach((v) => expect(css).toContain(v));
+  });
+});