feat(dashboard): add template listing and pull APIs

[sarkarshrayan2-max]

Summary

Implements the backend portion of #246 for downloading Fenn templates through the dashboard.

Changes

• adds GET /api/templates to list available public templates
• adds POST /api/templates/pull to download a selected template
• supports target-directory selection
• supports force-overwrite behaviour
• returns structured success and error responses
• extracts reusable template listing and pull logic from the CLI
• preserves existing CLI behaviour
• adds tests using mocked GitHub responses and temporary directories

Endpoints

List templates

GET /api/templates

Example response:

{"templates": ["base", "chatbot"], "total": 2}

Pull a template

POST /api/templates/pull

Example request:

{"template": "base", "path": "/path/to/project", "force": false}

Example response:

{"template": "base", "path": "/path/to/project", "downloaded": true}

Testing

• Ruff formatting passed
• Ruff checks passed
• Dashboard and pull tests passed: 57 passed

Scope

This PR implements the backend APIs. Dashboard UI integration and template-file preview can be handled as a follow-up.

Refs #246

[sarkarshrayan2-max]

I opened #258 with the proposed backend implementation.

It adds the template-listing and template-pull APIs, target-directory and force options, structured errors, reusable CLI logic, and tests. The dashboard UI and file preview remain follow-up work.

[ApusBerliozi]

Hey @sarkarshrayan2-max!

Everything looks fine, please fix 2 comments in the review above and I will merge your PR. Thx 4 your work :)

[ApusBerliozi]

Thx a lot for your fixes @sarkarshrayan2-max!

Unfortunately, there is currently a merge conflict, since we had another PR related to dashboard :(

Sorry for that. Please fix it, and we will proceed