Add editable graduation dates

[OwenTanzer]

Closes #31

What changed

• prompts for a graduation date before the existing Mark Graduated action runs, defaulting to today
• allows the date to be corrected later without removing Graduated status or changing the frozen 100% completion state
• rejects malformed and future graduation dates in both the UI and data store
• stores graduation as a canonical YYYY-MM-DD lifecycle date while preserving update and completion timestamps as audit history
• migrates legacy ISO graduation timestamps to the same local calendar date users previously saw
• renders corrected dates consistently on the dog profile and Trainer History, without timezone shifts
• lets legacy graduated dogs with no stored date add one
• adds regression coverage for normalization, calendar validation, and timezone-safe display

Why

Graduation was already a frozen lifecycle state, but its date was an uneditable audit timestamp. Trainers need to enter the actual graduation day and correct it later without undoing graduation or changing completion history.

Validation

• npm test (10 tests pass)
• npx tsc -b
• npm run lint (passes with three pre-existing warnings)
• npm --prefix worker run typecheck
• git diff --check