feat(webserver): add select-all checkbox to file tables in default template - #212

…mplate

Add a master checkbox in the table header of the Downloads, Search and
Shared pages that toggles every row checkbox at once, instead of having
to tick files one by one.

- Add a JS selectAll(check) helper to each page that walks all
  input[type=checkbox] in the document and sets them to check.checked.
- Replace the empty header cell with <th class="al-left"> containing the
  master checkbox (onclick="selectAll(this)"). The al-left class matches
  the left alignment of the row checkbox cells, which <th> would
  otherwise center.
- Name the master checkbox selectAllFiles (not a 32-char hash) so the
  command-processing loop, which only acts on 32-char file-hash names,
  ignores it automatically.

No backend changes; all logic is client-side.