[codex] Fix web jump image updates

[mickaelbegon]

Summary

This PR fixes the web behavior where selecting a different jump updated the numeric values but did not reliably update the displayed jump image.

Root Cause

Changing the selected jump calls Biomechanics.setValues(...) to reset the model. One field, _finalInertia, was declared as late final, so the second assignment raised a LateInitializationError. In the web app this interrupted the jump-change flow and left the UI in a partially updated state.

The image widget also had no asset-specific identity, so the background image replacement relied on Flutter reusing and updating the existing widget correctly.

Changes

• Make finalInertia mutable like the other editable biomechanics values, allowing jump changes to reset the full model.
• Key the background image widget and Image.asset with the selected asset path so the visible image follows the selected jump.
• Skip the landscape orientation lock on web to avoid web startup/orientation instability.
• Update web/index.html to use the current Flutter web bootstrap script.
• Add unit/widget tests for repeated biomechanics resets and background image asset changes.

Validation

• flutter test passed: 14 tests
• flutter analyze passed with no issues
• flutter build web --release succeeded
• Manual browser check on http://localhost:60205/#/jump-screen: Axel, Lutz, and Somersault all switch images and values without new console errors

Note: test the web app through an HTTP server URL, not by opening web/index.html directly with file://, because Flutter web assets/bootstrap are served differently.