feat: "Save data template" action emitting an example input file

[ahmadomira]

Summary

Help users format their input data correctly by example. Add a "Save template…" menu/toolbar action that writes a small example CSV/TXT file demonstrating the reader-accepted input format — column headers exactly as the app's readers expect, with 2–3 replicas and a few concentration→signal rows. This is an I/O-level, assay-agnostic convenience; the goal is to show a valid format, not to encode any assay.

Source

Reported via the fitting-app feedback channel.

Acceptance criteria

• A "Save template…" action writes an example input file to a user-chosen location.
• The emitted example loads cleanly through the app's existing reader(s) — it round-trips.
• Headers exactly match what core/io readers accept.
• Test that the emitted template is parseable by the loader; uv run ruff check / format clean.

Scope & file boundaries

Package C (bundled with the DBA simulator unit; they share only main_window.py in one session). Touch:

• gui/main_window.py (action)
• new example asset / small generator (and a small core/io helper only if needed)

Must NOT modify plotting widgets, fitting_session.py, or the fit pipeline.

Parallelization

Package C — runs in parallel with packages A, B, D.