Add multi-filter builder UI (frontend only)

Description:
Enhance the frontend so users can add or remove multiple filter rows.
Each filter row should map to the existing FilterCondition model (column, op, value) and be included in the JSON body sent to /process/csv.

Suggested files to edit:

index.html — add UI controls to dynamically add/remove filter rows and wire them into the existing JSON builder logic

Acceptance criteria:

User can add/remove N filters in the UI

Each filter becomes an entry in selection.filters within the JSON request sent by the UI

The backend already supports multiple filters, so no backend changes are required

Basic UI validation ensures each filter row has both a column and a value before inclusion