Standardize filter rows and add tag selection to the code page

Standardize filter rows and add tag selection to the code page

The code page kept its branch picker in the header actions, had no tag picker,
and left every list page to shape its own filters: search inputs ranged from
10rem to 18rem, some selects had a visible label and some did not, and text
searches submitted through a Filter button, a Search button, or Enter alone
while selects sometimes waited for a click.

- One .filters row on issues, requests, activity, the code page and code
  search. One --search-w (18rem) for page and global search, a visible label on
  every filter select, auto-apply on select change, and one icon-and-Search
  button for text.
- The code page puts its search, branch picker and new tag picker on the shared
  row under the header. The tag picker lists tags newest first and leaves the
  branch picker unclaimed when a tag is open.
- data-autosubmit uses requestSubmit() so htmx swaps the filtered pane instead
  of a full page load.

Checks: ./vole workflow check; ./vole workflow run verify.