[Feature Request]: Update label casing to sentence case

### Guidelines

- [X] I have searched the issue tracker for [open](https://github.com/FreeTubeApp/FreeTube/issues?q=is%3Aopen+is%3Aissue) and [closed](https://github.com/FreeTubeApp/FreeTube/issues?q=is%3Aissue+is%3Aclosed) issues that are similar to the feature request I want to file, without success.
- [X] I have searched the [documentation](https://docs.freetubeapp.io/) for information that matches the description of the feature request I want to file, without success.
- [X] This issue contains only one feature request.

### Problem Description

Our labels throughout FreeTube vary between [sentence case and title case](https://www.grammarly.com/blog/title-case-sentence-case/). There is no consistency, and this can at times be mildly jarring visually.

### Proposed Solution

Standardize all labels in FreeTube under sentence case, the one preferred by [many style guides](https://uxdesign.cc/title-case-vs-sentence-case-in-ux-writing-212087192261) ([see discussion here as well](https://www.caseconverter.com/sentence-case-vs-title-case/)). We can do this either [programmatically](https://stackoverflow.com/questions/4322314/convert-uppercase-letter-to-lowercase-and-first-uppercase-in-sentence-using-css) or manually (which sounds terrible). If programmatically, apply to all elements descending from the root of the document - this should take one rule, and maybe getting rid of any `text-transform`s elsewhere if they clash with this vision. Any exceptions to this rule should be exceptionally rare. I don't think we can write exceptions for labels starting with a word that should be lowercase given our large amount of supported languages, but that's fine. Of course, this does not apply to modifying user-generated content, unless we're talking about `Display Titles Without Excessive Capitalisation And Punctuation`, which @efb4f5ff-1298-471a-8973-3d47447115dc has rightly called out needs to be retested with the implementation of this feature.

Test modals and sections throughout the app to confirm that there are no issues with visually portraying the hierarchy with this change. If there are any, use additional styling like bolding and minor size differentials to resolve them.

### Alternatives Considered

None

### Issue Labels

visual improvement

### Additional Information

_No response_

Start at the app's root styling and inspect existing text-transform rules, then review modals and sections throughout FreeTube for casing and hierarchy issues. Confirm that labels use sentence case without changing user-generated content, and that any hierarchy concerns are resolved with styling before testing the affected views.

Written by the indexing model from the issue text.