fix(settings): clear previous provider highlight on selection

[SH20RAJ]

Summary

Fixes provider selection behavior in Settings -> Providers so only one provider appears selected at a time.

Issue

Closes #58

Root Cause

Provider selection logic was overriding background color directly, while ProviderItem rendering is controlled by shader instance states for selected and hover. That mismatch allowed stale visual states to accumulate.

Changes

• Reset built-in provider items using selected = 0 and hover = 0
• Mark the chosen provider with selected = 1
• Keep custom provider selection and hover state aligned the same way
• Update dark-mode redraw path to preserve selected-state semantics for built-in and custom providers

GSoC Ideas Alignment

This contributes to MoFA Studio UX quality under the GSoC ideas list where mofa-studio is involved (Idea 2 / Studio HCI and observability surface quality).

Validation

• Static code review completed
• Could not run cargo checks in this environment because cargo is unavailable (command not found)

[SH20RAJ]

Checked status: this PR is pending maintainer review/merge from upstream maintainers. I’m available to address feedback quickly.