Improve Experiments UI for scalability with categorization and bulk management

[raftaar1191]

What problem does this address?

Users must manually check/uncheck each individual experiment checkbox. When there are multiple experiments in a section, it's tedious and time-consuming to toggle all of them at once. This is especially problematic for:

• Testing workflows – Quickly enabling all experiments to test new features
• Auditing/cleanup – Disabling all experiments before performance testing or site audits
• Future scalability – As more experiments are added, managing them individually becomes increasingly time-consuming

This feature would provide a quick way to enable/disable all experiments in a category without clicking each checkbox individually.

What is your proposed solution?

Add a "select all" checkbox at the top of each experiments section header:

1. Editor Experiments section – add a master checkbox in the header to select/deselect all editor experiments at once
2. Admin Experiments section – add a master checkbox in the header to select/deselect all admin experiments at once

The checkbox should:

• Toggle all checkboxes in that section when clicked
• Show three-state behavior (all checked, all unchecked, mixed state) following WordPress conventions
• Be positioned prominently in the section header, similar to the pattern used in:
  • WordPress Posts list table

[jeffpaul]

This seems like something that would be good to include in the work happening in #153.