### Check for existing issues - [x] I have searched the existing issues.

### What happened?

The proxy `/user/daily/activity` endpoint (and the Usage page in the UI that consumes it) returns spend and request counts that are larger than what is actually in `LiteLLM_DailyUserSpend` for the user-requested date range, when the caller is in a non-UTC timezone.

**Customer context:** A customer was comparing LiteLLM-tracked spend for a single GCP project against GCP Billing for May 11, 2026 to validate spend tracking. The LiteLLM Usage page (Project Spend, "May 11 → May 11") showed **$8.7824** across **176 requests**. A direct SQL query against `LiteLLM_DailyUserSpend` for May 11 only showed **$0.99** across **38 calls**, which matches GCP Billing closely (~$1.10 after savings). So spend capture is correct — the daily-activity endpoint is the source of the inflated number, and it eroded customer trust in the dashboard. Without doing the SQL query ourselves, the customer would have walked away believing LiteLLM was off by ~8x.

### Root cause (suspected)

In `litellm/proxy/management_endpoints/common_daily_activity.py`:

- `_adjust_dates_for_timezone` (line 382) intentionally expands th…