Feature 1 — Overdue follow-ups panel

Blocked by issue 002. The panel is built on top of overdue_followups, which compares dates as text today. Fix that bug first (its test in tests/test_bugs.py must be green), otherwise the panel will show the wrong leads.

Why

As a salesperson, I want to see at a glance which follow-ups I have missed and by how long, so that I chase the oldest ones first instead of scrolling through the whole pipeline.

What

• [ ] logic.days_overdue(followup_on, today) returns the number of whole days between a stored follow-up date and today. It accepts dates with and without zero padding (2026-9-5 and 2026-09-05 are the same day).
• [ ] logic.overdue_report(today=None) returns the overdue open leads, oldest first. Each item is the lead dict plus a days_overdue key with the number from days_overdue.
• [ ] Leads whose follow-up is today or later, and leads that are won or lost, are not in the report.
• [ ] The Pipeline page has a section titled Overdue follow-ups (a subheader with exactly that text) listing, for each item of the report, the name, company, follow-up date and the days overdue.

Out of scope

• Sending reminders or emails.
• Changing the follow-up date from the panel.
• A per-stage breakdown of overdue leads.

Done when

• pytest tests/test_feature_1.py is green, without changing the tests.
• The panel shows up on the Pipeline page and matches what you count by hand in data/leads.csv.
• The work is on its own branch, with a commit message that says what you built.