feat(ctrl): improve template errors and GitHub URL handling

[akashgreninja]

closes #

📑 Description

✅ Checks

• [x] I have tested my code (provide screenshots or screen recordings of a working solution)
• [x] I have performed a self-review of my code No breaking changes; behavior unchanged apart from error text.

Feature 1 1)Normalize GitHub repo URLs by stripping .git.

https://github.com/user-attachments/assets/edbfbb61-128b-4ea7-ac8c-595eccdf93ac

2. Clear, actionable errors for invalid path (must be chart root). Add User-Agent and safe Accept for GitHub tarball fetch.

3. Bad commit/tag → clear ref-not-found message.

4. Private repo checks

ℹ Additional context

[akashgreninja]

@petar-cvit please can i get a review ?