Fix/palette management discoverability

[pranvinit]

What?

Closes #79897

Renames “Show details” to “Manage colors” or “Manage gradients” in the palette options menu.

Why?

“Show details” does not clearly communicate that users can edit or delete custom palette entries.

How?

Adds palette-specific labels and updates the related component tests.

Testing Instructions

1. Open Site Editor → Styles → Colors → Edit palette.
2. Add a custom color if none exists, then select Done.
3. Open the Custom palette’s three-dot menu.
4. Confirm Manage colors appears.
5. Select the Gradient tab and confirm Manage gradients appears for custom gradients.
6. Confirm the options open the corresponding editing controls.

Testing Instructions for Keyboard

1. Navigate through the steps above using Tab, Shift+Tab, Enter, and arrow keys.
2. Confirm the menu items are reachable and correctly announced.
3. Confirm focus moves into the palette controls after selecting the manage option.

Use of AI Tools

OpenAI Codex assisted with tests, and PR documentation. The changes were implemented, reviewed and tested by the contributor.