refactor: update aura accent surface color computation

[jouni]

Fine tune the accent surface color computation. Align it better with the regular (neutral) surface color. Fix the support for --aura-surface-opacity.

Before & after (light):

Before & after (dark):

Accent surface opacity

The main fix is that the accent surface color now responds to the --aura-surface-opacity property as expected. Before, the accent surface color was always slightly semi-transparent, even with --aura-surface-opacity: 1. This becomes an issue after #10989 is merged, when notification overlays start to use the accent surface color.

This is not applied in Safari 17 (limitation/bug in relative color syntax), and you can see the accent surface colors still being very slightly semi-transparent, especially the lower levels (surface opacity is 1):

We should probably document this limitation/issue.

Component theme tweaks

A few component surface levels and opacities are adjusted at the same time, to align them better together while keeping their appearance close to what they were before.

Button, before & after:

Tabs, before & after:

For the selected tab, I wanted to make the surface level higher, so it’s closer to white in light mode, making it less "dirty" against the background.

Side Nav, before & after:

[web-padawan]

LGTM. Updated visual tests screenshots for button, drawer-toggle and upload-button.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[vaadin-bot]

Hi @jouni and @web-padawan, when i performed cherry-pick to this commit to 25.0, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick cdb86d7
error: could not apply cdb86d7... refactor: update aura accent surface color computation (#11038)
hint: After resolving the conflicts, mark them with
hint: "git add/rm ", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".