Add ability to ask AI questions across multiple uploaded sources simultaneously

Currently, users can ask AI questions based on only one source at a time (PDF, Epub, link, etc.). This limits the application's potential for cross-document analysis and comparison.

**Feature request:**
Enable support for asking questions across multiple uploaded sources at once, similar to how it works in NotebookLM (where questions can reference an entire library of documents).

**Expected behavior:**
User selects 2 or more uploaded sources (files, links, etc.)

Enters a single question

AI analyzes all selected materials and returns a unified answer based on them

**Example user prompts:**
“Compare the approaches in these two PDFs”

“What are the key differences between these documents?”

“What common ideas are shared across the three uploaded files?”