Drop Train All workflow from WASM app surface

[Copilot]

“Train All” is disproportionately expensive in the browser/WASM runtime and does not fit the app’s execution model. This PR removes that path and keeps training scoped to explicitly selected models.

• UI: remove bulk-train affordance

  • Removed the Train All toolbar button from frontend/index.html.

• Frontend behavior: remove train-all code path

  • Deleted trainAllModels() from frontend/js/app.js.
  • Removed trainAllBtn state toggling from updateTrainButtons() so only Train Selected is controlled.

• Python bridge/API surface: prune unused endpoint

  • Removed train_all(...) from frontend/py/learner.py to match the new UI capabilities and reduce unnecessary backend surface.

• Internal docs alignment

  • Updated CLAUDE.md function list to no longer reference train_all.

// before: toggled both train buttons
document.getElementById('trainBtn').disabled = !(hasData && hasTarget && hasFeatures && hasSelectedModels);
document.getElementById('trainAllBtn').disabled = !(hasData && hasTarget && hasFeatures);

// after: only selected-model training remains
document.getElementById('trainBtn').disabled = !(hasData && hasTarget && hasFeatures && hasSelectedModels);