Use local dates instead of UTC day keys in profile and streak summaries

[getOffIt]

Summary

Profile and streak calculations currently derive day keys from toISOString(), which uses UTC rather than the learner's local day.

Problem

src/pages/profileProgressSummary.ts truncates date.toISOString() to YYYY-MM-DD. That shifts late-evening local activity into the next day for users west of UTC, and can shift early-morning activity for users east of UTC.

Impact

"Today", daily summaries, and streak calculations can be wrong even when the raw attempt timestamps are correct.

Relevant code

• src/pages/profileProgressSummary.ts:24-25
• other call sites using the same UTC truncation pattern

Expected behavior

Day-based reporting should use the user's local day, or an explicitly chosen application timezone.

Suggested direction

• Introduce a single helper for date-key generation based on local calendar dates.
• Replace direct toISOString().split('T')[0] usage in profile and streak code.
• Add tests around boundary times near local midnight.

[getOffIt]

assume local time is London, take daylight savings into consideration when needed