[DWAA1660]

What and why

Fixes #148. A new page's only box holds its date line, and container_content_height deliberately draws that box INITIAL_PAGE_LINES tall, which is the big outlined area the issue shows. The date paragraph is the only widget inside it, though, so a click anywhere below the date hit nothing. No caret appeared and typed text was lost. Only clicking the date itself (then Enter) worked.

containers::show now gives that blank area under the date its own click target, but only for a box whose single block is the page stamp (pageedit::is_page_stamp). A click there inserts an empty paragraph after the date in the same box and puts the caret on it, mirroring the existing handler that fills an empty box (start_in). Once the box holds more than the date it sizes to its content as before, so this only affects a fresh page.

How it was checked

• New UI test a_click_under_a_new_pages_date_starts_writing, for HTML and CMGFM pages created with node.create:
  • a click just under the date, and one further down, each give a caret
  • the typed TEST EMPTY AREA is in the saved page
  • Fails without the fix (caret: null, as in the issue), and passes with it
• cargo test -p notecraft-ui-egui: all pass (302 unit, 203 UI)
• cargo fmt --all -- --check, cargo clippy -p notecraft-ui-egui --all-targets -- -D warnings: clean

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