Add relational algebra mode

[Edwinexd]

Adds a relational algebra mode. RA expressions are transpiled to SQL and run against the same engine, so results validate against the existing question pool.

• Operators: selection, projection, rename, cross product, natural join, theta join, union, intersection, difference, division, distinct, grouping with aggregates, assignment.
• Projection follows set semantics (emits DISTINCT); distinct stays as an explicit operator.
• RA editor: syntax highlighting, reference dialog, SQL/RA mode toggle, preview of the generated SQL, PNG export.
• Mode-aware progress tracking, separate from SQL mode.
• License attribution dialog with generated dependency listing.
• Merged up to date with master (German); RA strings added to the en/sv/de packs.

Licensing will likely change before merging.

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[Edwinexd]

Note: Licensing will most likely be changed before merging

[Edwinexd]

Done in c1394f5: ra-engine relicensed from BSL 1.1 to PolyForm Noncommercial 1.0.0 (noncommercial, attribution required). Core app stays GPL-3.0.