[feat] Multi-table import

[thoughtspile]

1. Add a "plus" FAB to add a new table, open file picker on click.
2. When several tables are open, tabs at the top of the screen (above header) should switch between tables.
3. Pipelines for all the tables should be independent.
4. All the active files should be URL-persisted.
5. Show only one table at once, but keep the data (including evaluated pipelines) for all the tables in memory.

Additional context

This feature should prepare us for multi-table mode (join / concat). Features explicitly removed from MVP scope:

• Removing tables from the working set.
• Any multi-table pipelines
• Fast import actions (e.g. append to current table)