Automatically choose which collection to search when client doesn't specify

[nickthecook]

Both global search in the UI and /v1/search should have a technique for automatically selecting the collection to search.

Initial implementation

• change Collection creation so it goes through a form instead of being implicit create with edit-later
• add summary field with help text and examples that tell the user how to create a meaningful summary
• when a global search request is received, ask an LLM which Collection, based on their summaries, the query is most related to OR create a ChromaDB collection of summaries and use a similarity search
• use this new CollectionSelector service in UI global search
• use this new CollectionSelector service in API search