Put each repository tab's filters and search on one row

Closes #134.

What changed

Every repository tab now draws its filters and search as one row:

• Left: state/kind chips, labelled dropdowns, ref pickers — in that tab’s own order.
• Right: the search field, its button, and the clear action, in one .filters-search group that takes the row’s leftover space (margin-left: auto).

Pages

• Issues: state chips + Label + Assignee + Search on one line. The chips are inside the form now, so a search keeps the selected state; the htmx swap replaces the row with the server’s values.
• Changes: state chips + Search.
• Activity: kind chips + Actor + Assignee, with Clear on the right when a filter is active. The chips are inside the form so a picker change keeps the kind.
• Code: Branch and Tag pickers on the left, Search on the right (the search form moved after the ref pickers).
• Code search and global search: the same .filters-search group, so a lone search sits where every other page’s search sits.
• Runs, releases, compare: unchanged; they have no search, and runs’ chips are already one row.

Responsive

The row keeps wrapping at medium widths and stacks on a phone (max-width: 720px). The search stays one line there: the field flexes and the button stays beside it.

Tests

TestFilterRowPutsFiltersBeforeSearch checks every surface: a .filters row exists, a .filters-search group is the last thing in it, chips/labelled filters come before it, and the tabs that own chips keep them inside the row. TestFilterRowsShareOneShape and TestCodeBrowserRefPickers still pass.

gofmt, go vet, the link check, the full -race suite and vole workflow run verify are green.