fix: clear previous provider highlight on new selection

[Pavankumar07s]

📋 Summary

Fix the "sticky highlights" bug in the providers panel where clicking multiple provider items (e.g., OpenAI → DeepSeek → NVIDIA) accumulated highlight states instead of showing only the currently selected provider. The root cause was that select_provider_internal() and update_dark_mode() were not setting the shader's instance selected variable, which the ProviderItem pixel shader reads to render the selected background color. This PR adds selected: 0.0 / selected: 1.0 to the relevant apply_over() calls, ensuring only one provider is visually highlighted at a time.

🔗 Related Issues

Closes #58

Related to #58

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🧠 Context

The ProviderItem widget uses a custom Makepad shader with instance selected, instance hover, and instance dark_mode variables. The shader's pixel() function computes the background color from these instance variables:

fn pixel(self) -> vec4 {
    let base = mix(normal, hover_color, self.hover);
    return mix(base, selected_color, self.selected);
}

However, select_provider_internal() was only setting draw_bg: { color: (...) } — a property the shader never reads. This meant the shader's selected instance variable was never cleared on previously-selected items, causing all clicked items to stay highlighted. Custom provider items were already correctly using selected: 0.0/1.0, so only the four built-in providers (OpenAI, DeepSeek, Alibaba Cloud, NVIDIA) were affected.

🛠️ Changes

-Added selected: 0.0, hover: 0.0 to the reset loop in select_provider_internal() so all built-in items have their shader state cleared before the new selection
-Added selected: 1.0 to each built-in provider's selection branch so the shader renders the selected background
-Added selected: (selected_val) to the update_dark_mode() loop so dark mode toggling preserves correct selection state
-Added 3 unit tests for the ProvidersPanelAction enum (follows existing #[cfg(test)] module pattern)

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🧪 How you Tested

1. cargo check -p mofa-settings — compiles with no new warnings
2. cargo test -p mofa-settings — 27 tests pass, 0 failures
3. cargo clippy -p mofa-settings — no new warnings (all pre-existing)
4. Manual testing: cargo run --bin mofa-studio → Settings → Providers panel:
   Click OpenAI → only OpenAI highlights
   Click DeepSeek → only DeepSeek highlights, OpenAI clears
   Click NVIDIA → only NVIDIA highlights, DeepSeek clears
   Toggle dark mode → selected provider stays correctly highlighted

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📸 Screenshots / Logs (if applicable)

Screencast.From.2026-03-05.00-25-45.webm

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⚠️ Breaking Changes

• No breaking changes
• Breaking change (describe below)

If breaking:

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🧹 Checklist

Code Quality

• Code follows Rust idioms and project conventions
• cargo fmt run
• cargo clippy passes without warnings

Testing

• Tests added/updated
• cargo test passes locally without any error

Documentation

• Public APIs documented
• README / docs updated (if needed)

PR Hygiene

• PR is small and focused (one logical change)
• Branch is up to date with main
• No unrelated commits
• Commit messages explain why, not only what

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🚀 Deployment Notes (if applicable)

No migrations, config changes, or environment variable changes required. Drop-in fix.

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🧩 Additional Notes for Reviewers

The fix preserves all existing color logic (color: (normal_color) / color: (selected_color)) exactly as-is. The only additions are the selected and hover instance variables in the same apply_over() calls, which the shader's pixel() function actually reads. This is the minimal change needed to fix the glitch without altering any design decisions around colors or theming.

[Pavankumar07s]

@BH3GEI please take a look!