Interactive fit dialogs: show the model formula and evaluate models through Sigima #9

Context

DataLab 1.3.0 users report that the interactive fit dialogs give no indication of the formula being fitted: the parameter names ("Amplitude", "Scale factor", "Width factor", "Base line" for the Planckian fit) cannot be related to a model without reading sigima/tools/signal/fitting.py.

Problems

1. No formula in the dialog (enhancement): none of the dialogs created by sigimax.widgets.fitdialog displays the model equation, and parameter labels do not show the symbols used in the documentation (A, x₀, σ, y₀…).
2. Exponential dialog uses its own model (bug): exponential_fit evaluates a local modelfunc (a * exp(b * x) + c, no clipping) while the stored fit_params are later re-evaluated by sigima.tools.signal.fitting.evaluate_fit, which uses ExponentialFitComputer.evaluate. Both must use the same model.
3. The Planckian and exponential dialogs inherit the initial guesses and bounds from Sigima, which are being fixed in Planckian and exponential fits depend on data units; Planck signal and domain-based images lack axis information Sigima#74.

Proposed changes

• guifit(..., formula=None): when provided, show the formula as a selectable rich-text label in the FitDialog.
• Every per-type dialog passes the corresponding Sigima FitComputer.FORMULA (added in Planckian and exponential fits depend on data units; Planck signal and domain-based images lack axis information Sigima#74) and labels its parameters with their symbol, e.g. "Amplitude (A)", "Scale factor (x₀)".
• exponential_fit: evaluate with ExponentialFitComputer.evaluate and align the parameter ranges with Sigima.
• Update tests/widgets/test_fitdialog.py and French translations.

These changes go in the direction of #3 (computation in Sigima as single source of truth, per-type UI presentation) and do not conflict with it.

Related

• Refactor sigimax.widgets.fitdialog for parameter injection and UI/computation separation #3, Planckian and exponential fits depend on data units; Planck signal and domain-based images lack axis information Sigima#74, Wrong automatic Planckian fit, missing Planck axis information and synthetic images shown in pixels DataLab#400