improve the compose/confirm screens: recipient, full message, overflow arrows

[roykollensvendsen]

Three small UI improvements around composing and confirming a message.

1. Show the recipient address on confirm

So you can verify who a message is going to before submitting. For a single recipient — a one-off message (msg_recipient) or a thread reply (the thread's peer) — the confirm prompt's first line shows To <address>. Channels/groups have no single address, so they keep the plain separator. Reuses the existing fit helper.

2. Show every line of a multi-line message on confirm

The preview used to show only the first line. It now shows all lines (capped, with a dim +N more lines / char-count summary), and the overlay grows to fit — render_main_panel sizes it from the same confirm_preview helper the renderer uses, so the height always matches the content. Single-line messages look the same as before.

3. Show overflow arrows in the composer

When a message has more lines than fit in the input area, the composer scrolls but used to give no hint that lines were hidden. Now the gutter shows ↑ on the top visible line when lines are scrolled off above, and ↓ on the bottom line when there are more below — the vertical counterpart to the existing … for an over-long single line. No extra height used.

Tests

e2e harness tests for all three: recipient shown on confirm, a two-line message shows both lines, and the composer shows the up/down arrows when lines are hidden. cargo fmt --check, cargo clippy --all-targets -- -D warnings, and cargo test all pass.