Pressing TAB after focusing Theme applies theme instead of moving focus to next item

### Jenkins and plugins versions report

Environment

Jenkins: 2.552-SNAPSHOT
Theme Manager Plugin
Dark Theme Plugin
OS: Ubuntu 22.04
Browser:

### What Operating System are you using (both controller, and any agents involved in the problem)?

Controller: Ubuntu 22.04
Agents: None
Browser: Firefox 147.0 (Linux)

### Reproduction steps

1. Open the account menu using keyboard.
2. Navigate to the Theme option (using arrow keys or TAB).
3. When the Theme option is focused, press TAB.
4. Observe that the theme is immediately applied.

### Expected Results

Pressing TAB while the Theme option is focused should move focus to the next focusable item in the dropdown.
The theme should only be applied after explicit activation using ENTER or SPACE, not when focus changes.

### Actual Results

When pressing TAB while the Theme option is focused, the theme is immediately applied instead of moving focus to the next item.
The theme change occurs due to focus movement, without pressing ENTER or SPACE.

### Anything else?

_No response_

### Are you interested in contributing a fix?

Yes

Reproduce the account-menu interaction in Firefox on Ubuntu 22.04, focusing the Theme option and pressing TAB. Trace the Theme option's focus and activation handling in the plugin UI. Done means TAB moves to the next focusable item without applying a theme, while ENTER or SPACE still applies it.

Written by the indexing model from the issue text.