feat: add optional collection field to snippets

[AdvaitVarhade]

Resolves #23.

This PR introduces an optional collection field to the snippet schema to allow grouping beyond tags.

Changes included:

• Added collection to the underlying schema (make_snippet in models.py) and WTForms (forms.py).
• Added a "Collection" field to the Add and Edit snippet forms.
• Implemented a sidebar/dropdown collection filter on the home page, similar to the existing language filter.
• Displayed a collection badge next to the language badge on snippet cards and individual snippet views.
• Added unit test coverage in tests/test_routes.py.