PDF viewer improvements

Here are a few ideas to improve the PDF viewer:

1. It would be nice to be able to zoom out more in the PDF viewer, like, up to 50%. This would allow you to see more music at once without having to scroll too much if you have your tablet in landscape mode.
2. Being able to take notes in PDF files. This would help a lot when using this app for practice because you could mark difficult passages for later practice or add hints on how to play correctly.
3. Scrolling forward by tapping on the screen once. Maybe like this: tapping once to go forward half a page, tapping twice to go back half a page. Some apps also do this: The screen is divided in half, the upper part is the upper half of the first page, the lower part is the lower half of the fist page. When you tap, the upper part changes to the next page while the lower part stays the same. When you tap a second time, the lower part also changes to the second page. Like that, you can continue playing without having to adjust your eyes.

_Originally posted by @Maori1412 in https://github.com/Adithya-Jayan/MyRepertoirApp/discussions/224_

Locate the PDF viewer screen/component in the Flutter code and where its zoom level, gesture handlers, and page navigation are implemented. Then inspect how notes or annotations are modeled and stored in the app to determine where PDF-specific note data should live. Start by running the app and opening a PDF to confirm current behavior; completion means manual verification of 50% zoom, half-page tap-forward/back navigation, and ability to add/display notes on PDFs.

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