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

public/fonts/figlet-fonts/ANSI Regular.flf

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+flf2a$ 7 7 13 0 7 0 64 0
+Font Author: ?
+
+More Info:
+
+https://web.archive.org/web/20120819044459/http://www.roysac.com/thedrawfonts-tdf.asp
+
+FIGFont created with: http://patorjk.com/figfont-editor
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