fix: route simulateDate through getTodayInTZ (single date source of truth)

[krwg]

Summary

Dev date simulation and real calendar day helpers diverge. Make getTodayInTZ() the single source of truth so every habit/calendar/notification path respects Dev Mode date override automatically.

Problem

• dev-mode.js exposes simulateDate(dateString) and keeps override state separately.
• timezone.js getTodayInTZ() does not honor that override.
• New date-aware modules tend to call getTodayInTZ() and silently ignore simulation → unreliable Dev Mode bug reports.

Approach

In core/timezone.js:

• Private _devDateOverride (YYYY-MM-DD | null)
• __setDevDateOverride / __clearDevDateOverride
• getTodayInTZ() returns override when set

dev-mode.js simulateDate calls those helpers instead of a parallel path.

Acceptance

• Habits, analytics, calendar, native notifications all follow simulated day via getTodayInTZ
• Clearing override restores real TZ day
• No duplicate “today” helpers for production paths