Standardize capitalization across the Mothra UI

Capitalization is currently inconsistent across the Mothra interface, especially on the project page. For example:

- Page titles, navigation items, and actions are entirely lowercase, e.g., "mothra", "hello", "my projects", "begin."
- Section headings use mixed capitalization, such as “CantusDB Text settings” and “Glyph Classifier settings.”
- Some paragraph text begins with a lowercase letter—for example, the instructions below the “begin” button—while paragraphs on the walkthrough page begin with uppercase letters.
- Other labels appear as “training data,” “PRESETS,” and “Choose Files.”

Could we agree on a consistent capitalization convention for the UI?

One option would be to use Title Case for page and section headings, and sentence case for labels, descriptions, and actions, while preserving proper names and acronyms such as CantusDB, OCR, and XML.