fix(core): improve app icon contrast in the app menu

[pringelmann]

• Resolves: Improve contrast of app icons #62728

Summary

Before	After

App icons in the waffle menu were hard to tell apart, so the colours are flipped: pale primary tint for the circle, saturated primary for the glyph. Shine and drop shadow are gone, circle and glyph are slightly bigger, and the circle scales on hover.

-> Preview link <-

The glyph is painted through a CSS mask. App icons ship a hardcoded fill, so currentColor never applies and the old filter: invert() could only flip black and white, which is why an arbitrary theme color wasn't possible before.

Examples:

Light	Dark

Dark themes get no bevel. Their circle sits too close to black to darken any further, and mixing toward the primary brightened it into a glow instead.

New accessibility tests rebuild the lightest stop of both gradients and hold the contrast at 3:1 across default, dark and both high-contrast themes.

Other changes

AppIcon is shared with unified search results, so app results there change too:

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI (fiddly CSS, accessibility tests)

[pringelmann]

/backport to stable34