Text search

It should be possible to filter documents by presence of query word in their text content.
For this, it's necessary to implement:
1. Text-layer extraction from PDF and other text-based documents.
This must happen during folder indexing.
2. New button or menu option in Resources Grid screen, which displays text input box.
The string from the input should be searched in text layers of all resources.
Matched resources must be displayed.

Start by locating the folder-indexing entry point and the Resources Grid screen mentioned in the issue. Trace how indexed resource metadata is stored and how the grid applies filters. Done means text can be extracted during indexing and a query option displays every resource whose text layer contains the query.

Written by the indexing model from the issue text.