Allow logging multiple doses per day for multi-dose schedules

[snachodog]

Summary

• Today screen now allows logging up to the expected number of doses per day for twice-daily and three-times-daily schedules, instead of capping every schedule at one dose ("Taken 2 of 3 today" progress with per-dose timestamps and undo, same pattern as as-needed meds)
• Fixes a crash where a FastAPI validation error (422) rendered its raw detail array directly as a React child instead of a message, via a shared apiErrorMessage helper used across Settings.jsx
• Disables "Add medication" when no person exists/selected, preventing a guaranteed validation failure (person_id: null)

Closes #114

Test plan

• Manually verified on Today screen: added a med with "Three Times Daily" schedule, confirmed "Took it" can be tapped 3 times with progress shown, then disables
• Verified single-dose schedules (morning/evening/etc.) unchanged
• Verified as-needed meds unaffected
• npx vite build clean after each change
• Manually triggered a validation error and confirmed a readable message now shows instead of a crash
• Verified "Add medication" is disabled with no person selected