Remove absolute positioning from the header search input

[pringelmann]

The header search trigger is position: absolute in the header. That breaks tab order (DOM position no longer matches visual position) and is fragile on narrow/mobile viewports.

Center it with flex layout in the header instead. No restyle, the look stays the same - this is just a correctness fix.

• input is centered without position: absolute
• tab order matches visual order
• no overlap/overflow at 320px and on mobile
• focus reaches the input in the expected sequence