feat: implement Figlet, Pastel, and Unit tools with a unified layout

- Add Figlet text converter with font selection and history
- Add Pastel color palette generator and manipulation suite
- Add comprehensive Units converter with category-based logic
- Introduce AppShell with Sidebar and Header for navigation
- Modernize theme system with CSS variables and new animations
- Update project configuration and dependencies

types/units/convert-units.d.ts

@@ -0,0 +1,30 @@
+declare module 'convert-units' {
+  export interface Unit {
+    abbr: string;
+    measure: string;
+    system: string;
+    singular: string;
+    plural: string;
+  }
+
+  export interface Converter {
+    from(unit: string): {
+      to(toUnit: string): number;
+      toBest(options?: { exclude?: string[]; cutOffNumber?: number }): {
+        val: number;
+        unit: string;
+        singular: string;
+        plural: string;
+      };
+      getUnit(): Unit;
+      possibilities(): string[];
+    };
+    measures(): string[];
+    possibilities(measure?: string): string[];
+    describe(unit: string): Unit;
+  }
+
+  function convert(value?: number): Converter;
+
+  export default convert;
+}