Grow the prompt popup with wrapped text so the typed tail stays visible

[MJohnson459]

Size the prompt popup from its wrapped text, not its newlines

The text-prompt popup (Mode::Prompt — refine notes, questions, rejection
feedback) computed its height by counting \n-separated lines in the buffer.
Typed input never contains a newline, since Enter submits, so the box stayed
three rows tall while the Paragraph's Wrap { trim: false } wrapped anything
past ~70 characters below the single visible row — hiding both the tail being
typed and the cursor.

crates/voro/src/ui.rs now measures the popup with
Paragraph::line_count(inner_width) (the workspace already enables ratatui's
unstable-rendered-line-info), which counts wrapped rows and explicit newlines
alike, so pre-seeded multi-line RejectWork buffers (DESIGN.md §11c) still size
correctly. Width (72) and the 3..=20 height clamp are unchanged. When the
wrapped text exceeds the clamp the paragraph scrolls by the overflow so the
bottom — the cursor end — stays visible, rather than letting the same bug recur
for very long text. No keybinding or editing-model changes; Mode::LinkPr is
untouched.

Verified: two new TestBackend render tests in ui.rs — one asserts a
single-line buffer wider than the popup shows both its head and its
TAILMARK▏ tail, the other that a buffer wrapping past the height clamp shows
the tail and no longer the head. Both fail against the previous rendering and
pass after the change. cargo test --workspace (281 + 273 tests) and
cargo clippy --workspace --all-targets -- -D warnings pass; cargo fmt --all --check is clean.