feat: add date to active times display for cross-midnight sessions

[owieth]

Problem

firstActiveAt and lastActiveAt store only HH:mm strings. If the Mac stays on overnight or resumes from sleep the next day, the displayed range (e.g. 22:45 - 09:30) is ambiguous — there's no date context to tell the user these times span two different days.

Proposed Solution

1. EventMonitor.swift — change the time formatter from HH:mm to a full datetime format (e.g. yyyy-MM-dd'T'HH:mm) so the date is embedded in the stored value.

2. MenuBarViewModel.swift — add a formattedActiveTimeRange computed property with smart formatting:

   • Same day as today → HH:mm - HH:mm (existing look, no change)
   • Spans two different dates → MMM d, HH:mm - MMM d, HH:mm (e.g. Mar 29, 22:45 - Mar 30, 09:30)
   • Graceful fallback for legacy HH:mm values already in the database

3. MenuBarView.swift — use the new formatted property.

Notes

• No DB migration needed — first_active_at / last_active_at columns are plain text; the wider format is backwards-compatible.
• Legacy HH:mm values already stored will render as-is via fallback parsing.