Replace global experiments checkbox with enable/disable button

[Omcodes23]

What?

Closes #157

Replace the global experiments checkbox with an enable/disable button that auto-submits the settings form, reducing the number of actions needed to enable experiments from 4 to 3.

Why?

Currently, enabling AI Experiments requires:

1. Check the "Enable Experiments" checkbox
2. Click "Save Changes"
3. Check an individual experiment checkbox
4. Click "Save Changes" again

By replacing the global checkbox with a button that auto-submits, we reduce this to:

1. Click "Enable Experiments" button (auto-saves)
2. Check an individual experiment checkbox
3. Click "Save Changes"

This improves the user experience by making the global toggle more intuitive and reducing friction.

How?

• Replaced the "Enable Experiments" checkbox with a button that changes based on state:
  • When disabled: "Enable Experiments" with primary (blue) styling
  • When enabled: "Disable Experiments" with secondary (grey) styling
• Button uses a hidden input field to store the toggle state value
• Added inline JavaScript that updates the hidden field value and auto-submits the form on button click
• Added helper text below the button to clarify that clicking saves immediately
• Updated SCSS to support the new button-based toggle layout with flex-wrap
• Preserved the "Save Changes" button at the bottom of the page for individual experiment settings
• Updated E2E test helpers to work with the new button-based UI (click button instead of checkbox)

Testing Instructions

1. Navigate to Settings > AI Experiments (ensure you have valid AI credentials configured)
2. Verify the page shows "Enable Experiments" button with primary (blue) styling
3. Click the "Enable Experiments" button
4. Page should reload automatically with "Settings saved" success notice
5. Verify the button now shows "Disable Experiments" with secondary (grey) styling
6. Verify individual experiments are now enabled and can be toggled
7. Verify the "Save Changes" button at the bottom still works for individual experiment changes
8. Click "Disable Experiments" button
9. Page should reload with "Settings saved" success notice
10. Verify the button is back to "Enable Experiments" with primary styling
11. Verify individual experiments are disabled (greyed out)

Testing Instructions for Keyboard

1. Navigate to Settings > AI Experiments using keyboard (Tab)
2. Tab to the "Enable Experiments" button
3. Press Enter or Space to activate the button
4. After page reload, verify button text changed to "Disable Experiments"
5. Tab to individual experiment checkboxes and verify they are now focusable and editable
6. Tab to "Disable Experiments" button and press Enter or Space
7. After page reload, verify button text changed back to "Enable Experiments"
8. Verify all interactive elements maintain proper focus order and visual focus indicators

Screenshots or screencast

Before	After
Checkbox labeled "Enable Experiments" with separate "Save Changes" button required	"Enable Experiments" button (primary styling) that auto-saves when clicked
After checking: Same checkbox UI, manual save needed	After clicking: "Disable Experiments" button (secondary styling) with helper text explaining immediate save

[github-actions]

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Co-authored-by: Omcodes23 <omcodes23@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.45%. Comparing base (965498c) to head (6e60327).
⚠️ Report is 14 commits behind head on develop.

Files with missing lines	Patch %	Lines
includes/Settings/Settings_Page.php	0.00%	25 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #168      +/-   ##
=============================================
- Coverage      50.80%   50.45%   -0.36%     
- Complexity       375      379       +4     
=============================================
  Files             27       27              
  Lines           1978     1992      +14     
=============================================
  Hits            1005     1005              
- Misses           973      987      +14     

Flag	Coverage Δ
unit	50.45% <0.00%> (-0.36%)	⬇️

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[dkotter]

Overall this looks okay to me though we'll want to ensure this works fine with the changes in #153.

In addition, there are E2E failures here that will need fixed up before we merge this in

[Omcodes23]

@dkotter Now Works ?

[Omcodes23]

@dkotter @jeffpaul any updates ?

[jeffpaul]

@Omcodes23 we're working to wrap up work on version 0.2.0, we'll get to this next as we start to work on 0.3.0.