fix(pantry): let the Add item form create a new section

[LeMyst]

Closes #628

Problem

Since 08233b7 (#293), the Section list in the pantry's Add item dialog only lists the sections already in pantry.conf once the pantry has an item. With a single [fridge] section, every new item had to go to the fridge.

Change

• The Section list keeps its current entries (the existing sections, or the defaults when the pantry is empty) and gains a last New section… choice. Picking it reveals a "New section name" field, which becomes required and takes focus. If the name is blank, the dialog shows an error instead of sending the request.
• Section names can now be typed freely, so the item update/remove requests encode the section in the URL (/api/pantry/Wine%20cellar/Merlot). The selector that runs after a removal escapes it with CSS.escape.
• pantry-section-new and pantry-section-new-name are added in all 9 locales.

The server already created a missing section on POST /api/pantry/add, so there is no Rust change.

Testing

• cargo fmt --check, cargo clippy --all-targets (no warnings), cargo test (all pass, including locales_test)
• Playwright (Chromium) against a pantry with only [fridge]:
  • The list shows fridge and New section…. The name field is hidden until "New section…" is picked.
  • Adding Merlot to "Wine cellar" writes ["Wine cellar"] to pantry.conf, and the page shows both sections.
  • Picking fridge still works, and the field stays hidden.
  • Removing Merlot calls DELETE /api/pantry/Wine%20cellar/Merlot, which returns 200.

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