feat: arrow-key pickers for setup, /backend, and /model

[nelopuchades]

Motivation

Setup and the /backend / /model flows still used type-a-number prompts:
print a list, ask for a digit, hope the user typed the right one. That works
for a handful of backends, but model lists (OpenRouter, local servers, etc.)
are long, easy to mis-type, and don't show what's currently selected while
you browse.

This change adds a shared interactive select menu and wires it into the
setup wizard and the slash-command pickers, so choosing a backend or model
feels like the rest of the terminal UI: arrows, Enter, cancel — with the
current choice marked and the title staying left-aligned.

Summary of changes

Shared picker (select_from_list)

• Add an interactive single-choice menu: ↑/↓ (or j/k), Enter to confirm,
  1–9 jump-and-confirm, q/Esc to cancel
• Window long lists (12 visible rows) with … N above / … +N more
• Highlight the focused row with the accent pointer; callers mark the live
  choice with *
• Park the cursor at column 0 after raw-mode line input (\r\n) and before
  the first select frame (\r) so the bold title/gutter don't start mid-line
  and jump left on the first arrow redraw

Setup wizard

• Backend, model, approval policy, and tool mode prompts use the shared menu
• Model step fetches /models when possible, builds a sorted menu (default /
  current override injected), and keeps a free-text fallback when the list is
  empty or unreachable

/backend and /model

• Replace numbered type-to-select (and the old model filter list) with the
  same arrow menu
• Cloud models still append catalog cost/context labels where known
• Free-text model entry remains available as the last menu option / fallback

Verified by

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test (input select-menu tests + suite)

Screenshots