Add support for file attachments / uploads

[bnaylor]

Iris should be able to process things beyond text files, depending on the supporting model.

• png, jpeg, etc
• pdf
• docx
• etc

1. Add an 'attachment' 📎 button somewhere near the input box.
2. Allow for drag & drop of files into the chat window
3. If possible, filter file types by model capability, and surface incompatibilities with a reasonable error message, eg ("this model doesn't support vision")

[bnaylor]

If possible, filter file types by model capability, and surface incompatibilities with a reasonable error message, eg ("this model doesn't support vision")

Hermes supports using different models for different things - for example, Rune uses DeepSeek as his main model but calls out to gemma4:12b on Ollama to do vision. We could go down that path instead.