Add split preview visualization to model session configuration

[Irozuku]

Summary

The session sidebar now draws the splits a configuration will produce, under the evaluation strategy selector. The figure reacts to the selected strategy, the selected splitter and its parameters, with a colored band per partition or fold.

Which figure to draw is read from the splitter, not guessed from its schema parameters. Every splitter declares a geometry, and every splitter and evaluation strategy carries a MultilingualString description that the sidebar prints next to the figure. A splitter whose shape has no renderer draws nothing rather than drawing a confident picture of a split that never happens.

Video_260911104205.mp4

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Type of Change

Check all that apply like this [x]:

• Backend change
• Frontend change
• CI / Workflow change
• Build / Packaging change
• Bug fix
• Documentation

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Changes (by file)

Backend, split shape:

• DashAI/back/splitters/base_splitter.py: adds GEOMETRY, defaulting to "unknown", and reports it from get_metadata.
• DashAI/back/splitters/holdout.py: PartitionSplitter declares "partitions", inherited by HoldoutSplitter.
• DashAI/back/splitters/fold_splitter.py: declares "blocked_folds", inherited by every k-fold family splitter.
• DashAI/back/splitters/temporal_holdout.py: declares "sequential_partitions".
• DashAI/back/splitters/rolling_origin.py: declares "expanding_window".

Backend, prose:

• DashAI/back/splitters/*.py: every registered splitter gains a DESCRIPTION in the five supported languages, saying what the split does and when to reach for it.
• DashAI/back/evaluation/holdout.py, cv.py, forecasting_holdout.py, forecasting_cv.py: same for the four evaluation strategies. The components API already runs responses through localize, so no new plumbing was needed.

Frontend:

• DashAI/front/src/utils/splitPreview.js: new. Pure geometry, no React. Turns a geometry plus the live parameters into rows of colored segments. Four renderers: single bar partitions, sequential partitions, blocked folds with a sliding validation block, and an expanding window whose train prefix grows by step and is scored on the next horizon rows.
• DashAI/front/src/components/models/modelSession/SplitPreview.jsx: new. Renders the rows, a legend with percentages, the strategy and splitter descriptions, and the reserved test band.
• DashAI/front/src/components/models/modelSession/SplitsCard.jsx: new. The card shell extracted out of SplitDatasetRows so both files can use it.
• DashAI/front/src/components/models/modelSession/SplitDatasetRows.jsx: renders the preview after the splitter selector, and resolves selectedStrategy once instead of calling findStrategy three times.
• DashAI/front/src/utils/i18n/locales/*/experiments.json: a splitPreview group with the labels the figure draws. The explanatory prose is not here; it comes from the components API.

Tests:

• tests/back/splitters/test_splitter_geometry.py: new. Pins that every registered splitter reports a shape the frontend has a renderer for, that the series splitters are never drawn as shuffled ones, and that geometry sits alongside what the metadata already carried.

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Testing

• Open a session, switch between Holdout and Cross Validation, change the splitter and edit n_splits, test_size, horizon and step. The figure should redraw on every keystroke. A rolling origin configuration that does not fit reports the same condition the backend raises on instead of drawing folds.