feat(shell-panel): add size override capability

[Elijbet]

Related Issue: #10568

Summary

Prototype: Standardize and expose the existing internal pattern as a public updateSize(size, axis) method backed by a shared useSizeOverride controller. Preserve one‑way token semantics, enable explicit reversible runtime sizing, reduce duplication, and clarify when to choose tokens vs imperative overrides.

Tests: Migrate vertical/horizontal resize E2E to browser mode; add isolated controller spec.
Docs: Added JSDoc (when to use vs tokens).

[jcfranco]

@Elijbet Can you confirm whether this is still a prototype or the feature intended to land? If it’s the latter, can you update the description to reflect this? Also, can you revisit the title to focus on the feature itself (i.e., no need to call out useSizeOverride as this is an implementation detail that can be covered in the PR description or the commit itself).

[Elijbet]

@Elijbet Can you confirm whether this is still a prototype or the feature intended to land? If it’s the latter, can you update the description to reflect this? Also, can you revisit the title to focus on the feature itself (i.e., no need to call out useSizeOverride as this is an implementation detail that can be covered in the PR description or the commit itself).

This is still a prototype - I want to get feedback from the team on whether this approach makes sense before finalizing it. Once we confirm the approach and I address any feedback, I'll update it to be merge-ready as the final feature.