Add selectedcontent button to select inputs

[Flo0807]

What

Adds a customizable-select trigger to every native <select> in Backpex:

<button>
  <selectedcontent></selectedcontent>
</button>

inserted as the first child of each <select>. Affected selects:

• Backpex.HTML.Form.input/1 (type="select") — the shared select input used by the select and belongs_to fields
• Backpex.Filters.Select.render_form/1 — the select filter
• Backpex.HTML.Resource.select_per_page/1 — the "items per page" picker

Why

Backpex already ships ::picker(select) utility classes (e.g. in the error styling of the select input), which only take effect under appearance: base-select. This change completes that direction: with the customizable-select rendering, <selectedcontent> mirrors the currently selected <option>'s content into the styleable trigger <button>, while the element stays a real native <select> — keyboard support, form submission and accessibility semantics are unchanged.

Browsers without customizable-select support simply ignore the <button> and fall back to the standard native <select>, so this is a progressive enhancement.

Notes for review

• The form select supports multiple={@multiple}. The <button>/<selectedcontent> pattern is meaningful for single-select comboboxes only; for multiple selects it is inert. I added it unconditionally per the change's intent — happy to guard it with :if={!@multiple} if preferred.

Verification (local)

• mix compile --warnings-as-errors --force ✅ (HEEx accepts <button> inside <select>)
• mix format --check-formatted ✅
• mix test ✅ 166 tests, 95 doctests, 0 failures
• mix credo ✅ no issues
• mix docs --warnings-as-errors ✅