Display fenn.yaml and main.py in dashboard

[sarkarshrayan2-max]

Parent issue

Part of #246.

Dependencies

Blocked by #258 and the dashboard template-download UI issue.

Description

Allow users to preview the main files of a downloaded Fenn template directly in the dashboard.

Requirements

• locate fenn.yaml in the downloaded template directory
• locate main.py in the downloaded template directory
• display both files as read-only previews
• preserve file formatting and line breaks
• show a clear message when either file is missing
• prevent reading files outside the selected template directory
• add syntax highlighting where practical

Acceptance criteria

• fenn.yaml can be displayed in the dashboard
• main.py can be displayed in the dashboard
• missing files do not crash the dashboard
• unsafe paths are rejected
• tests cover valid files, missing files, and path traversal attempts