fix: preserve leading zeros in HUC codes by casting to str

[Ehakl]

Fixes #40

Problem

The huc_cd value returned by load_site_data() in data_retrieval.py was not explicitly cast to a string. When the NWIS API returns HUC codes with leading zeros (e.g. 01080106 for site 01151500), pandas can silently cast the value to an integer, dropping the leading zero and producing 1080106.

Fix

Wrap resp['huc_cd'].iloc[0] in str() on line 301 of hydroshift/utils/data_retrieval.py to ensure the HUC code is always stored as a string, preserving any leading zeros.

This is a one-line change with no side effects — the value is only ever used for display in the site_summary.md template.