Flask learning is done. Today I built a complete mini Flask application from scratch: an expense tracker where users can register, log in, add expenses, filter them by category, and see a running total. Everything from days 78 to 84 came together in one day of building. Here's how it's architected and how it works. Why an Expense Tracker I wanted something small enough to finish in a day but real enough to actually use. An expense tracker hits that balance, it solves a genuine problem, it uses every Flask concept I've covered, and it's different enough from DevBoard and the other things I've built that it shows range rather than repetition. Structurally, it's similar to a to-do app: a list of items that belong to a user. What pushes it beyond tutorial territory is the fields on each expense, the category filter, and the running total. Those three things make it feel like a real tool rather than a practice exercise. What the App Does The app has two sides: authentication and expense management.…