bug: ion-select placeholder has wrong opacity value when labelPlacement="floating" #30402

Prerequisites

• I have read the Contributing Guidelines.
• I agree to follow the Code of Conduct.
• I have searched for existing issues that already report this problem, without success.

Ionic Framework Version

v8.x

Current Behavior

ion-selects with labelPlacement="floating" show their placeholders with full opacity, instead of the --ion-placeholder-opacity (default 0.6 in md mode)

(all selects are artifically styled as expanded in this example, but this issue is also visible when the placeholder is shown during normal use)

Expected Behavior

The placeholder should have the correct opacity (var(--placeholder-opacity)).

Steps to Reproduce

1. Open the reproduction URL
2. Click on any floating select to show its placeholder. Alternatively click the button "style as expanded" to artifically style all selects as expanded at the same time.

Code Reproduction URL

https://stackblitz.com/edit/pxhxnc4c-a6vaud9w

Ionic Info

Ionic:

Ionic CLI : 7.2.1
Ionic Framework : @ionic/angular 8.5.7
@angular-devkit/build-angular : 19.2.11
@angular-devkit/schematics : 19.2.11
@angular/cli : 19.2.11
@ionic/angular-toolkit : 12.2.0

Utility:

cordova-res : not installed globally
native-run : not installed globally

System:

NodeJS : v20.19.1 (/usr/local/bin/node)
npm : 10.8.2
OS : Linux 5.0

Additional Information

The CSS rule to unhide the placeholder for floating labels sets the opacity to the fixed value 1, instead of var(--placeholder-opacity).

There is also a related issue visible in this example, where the placeholder overlaps the label when the input is focused, see #30401.