Fix Pokédex region filters on mobile

[engelben]

Motivation

• Mobile generation pills were cramped and overlapped on narrow screens.
• Use a familiar, accessible mobile control while retaining quick-access pills on larger viewports.

Description

• Uses a full-width native generation selector below 640px and the existing pill controls from 640px upward.
• Prevents desktop pills from shrinking or overlapping while keeping their row horizontally scrollable.
• Keeps the URL as the generation source of truth, including browser Back/Forward navigation.
• Normalizes malformed, duplicate, and non-canonical generation parameters without discarding unrelated query parameters.
• Preserves unrelated query parameters when changing or clearing the generation.
• Adds accessible English and German labels plus selected-state semantics for the desktop controls.
• Updates the release version to 1.27.4.

Testing

• 91 frontend tests
• 283 backend tests, 1 skipped, and 20 subtests
• Production frontend build and version-consistency check
• Fresh frontend and backend Docker builds plus frontend container smoke test
• Chromium testing at 280px through 1024px, including the 639/640px breakpoint, keyboard input, rapid changes, refresh, malformed URLs, repeated Back/Forward, and resize/orientation changes

No page overflow, console errors, JavaScript errors, or failed browser requests were found.

[Git-Romer]

Thanks, @engelben, for this contribution. The native mobile selector solves the cramped and overlapping generation filters nicely while preserving the desktop pills.

During the final review, I added a few follow-up improvements:

• Integrated the current main branch.
• Normalized invalid, duplicate, and non-canonical generation URLs.
• Preserved unrelated URL parameters when changing generations.
• Improved accessible labeling and selected-state semantics.
• Added focused regression coverage for the selector and URL behavior.
• Updated the release version to 1.27.4.

The final desktop, mobile, navigation, build, and automated validation passes, and the PR is ready to merge.