Major Backend Rework

[Piyuuussshhh]

• ops.rs was the most fucked rust code anyone would've ever seen. Had to refactor and implement a better, scalable design.
• which is what this PR aims to do.

Advantages

1. Implementing Habits is easier than ever now.
2. The code is readable and modular.
3. Coupling and cohesion levels are better than ever. Earlier all db stuff was done in ops.rs, now its modularized.
4. Files and functions do what they are NAMED to do. Nothing less, nothing more.
5. Finally made me realise the importance of reading the documentation and understanding the features a framework offers T_T (if i'd known about State<'_, T>, why would I do what I did in the first place? That singleton was my best effort at mimicking state management.)

Cons

1. Exporting now takes a while because the pdf file has to be created BEFORE it can be saved using the file system dialog. Earlier, the pdf file was created when the user pressed the 'Save' button.
2. Took me the entirety of yesterday and made me suffer from imposter syndrome.

Stuff Left To Do In This PR:

1. I think many variables are marked as pub simply in todo.rs.
2. The database file needs to be named according to the user's operating system. It was implemented in the old code, I was too busy to do it in this version. Left as an exercise for @Arnax308 lmao.

[Piyuuussshhh]

Long export times are now fixed!