Cache Major Projects Form Until Submission

Prevents users from losing their progress if they accidentally refresh, close the page, or want to come back and finish the submission later.

Locate the Major Projects submission form and its Flask entry point, then trace how the form currently handles refreshes and submission. The issue names no files or tests, so first identify the relevant form flow and its existing test coverage. Done means entered progress survives an accidental refresh or leaving and returning, and the cached data is cleared after submission.

Written by the indexing model from the issue text.