Refactors and new technologies after v3-stable

[WhyAsh5114]

Is your feature request related to a problem? Please describe.

• The codebase suffers from a lot of monkey-patching problems for different use cases. This is because I didn't use any classes to create and handle complex objects and relied on just objects with types and utility functions. This doesn't scale as the app grows, so I'll eventually need to update the code to use the Builder pattern to create complex objects like ExerciseSplits, Mesocycles, and Workouts.
• I also think Svelte and SvelteKit might not be the best choice for the frameworks, when I started this project they were. But now that I tested out NextJS 15 with React 19 using server actions and partial prerendering, I think they've come a long way, they even eliminate the need for tRPC while still giving type-safety.
• There's also a better auth library called Better Auth that offers a lot more features with much less config, might move to that later.

Describe the solution you'd like

Eventually migrating to these new technologies and software patterns, is not necessary for v3-stable, but something to do after it is complete. This might even be v4 as the tech stack changes completely.