Collections/folders on top of tags

[byteofhoney]

Goal:

Add a broader grouping option above tags for organizing snippets.

User story:

As a user, I want to group snippets into collections so that I can organize beyond just tags.

Tasks:

• Add an optional collection field to the snippet schema
• Add a dropdown/sidebar filter by collection, similar to the language filter
• Decide (and document in the PR) whether a snippet can belong to more than one collection

Acceptance Criteria:

• Snippets can be assigned a collection when created or edited
• Filtering by collection shows only matching snippets
• Snippets with no collection assigned still display normally in the "all" view

[AdvaitVarhade]

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 subtle collection badge next to the language badge on snippet cards and individual snippet views.