fix(tui): show remote pairing in a full-height panel #1918

Problem

In v0.64.0, /remote opens an 18-row command panel. A pairing QR alone occupies about 29 rows, so it cannot be displayed in full and scanned. The Remote panel should occupy the terminal's full height.

Evidence

• User report: /remote is too short to scan its QR code.
• Merged remote PRs: feat(engine): add remote protocol v1 and identity-bound operations #1911, feat(engine): add the remote gateway foundation with pairing and devices #1912, feat(tui): bridge the running session to a remote owner link #1913, feat(engine): route shared sessions through the remote gateway #1914, fix(tui): describe the remote gateway as it now works #1915; release chore(ci): release v0.64.0 #1916.
• ui/modals/panel.rs: COMMAND_PANEL_HEIGHT = 18; render_command_panel anchors above the composer.
• remote_gateway/qr.rs: pairing codes include a four-module quiet zone and half-block rows, about 29 terminal rows.
• paint_panel_line_backgrounds overwrites explicitly white QR span backgrounds with the panel colour.
• Live rustcode remote status on 2026-10-09: v0.64.0 gateway listens on 127.0.0.1:17879, one shared session, zero paired devices. Session: 01a12118cc75-7000-a423-1991-1991685c0034 (session files under ~/.config/rustcode/sessions/2026/10/09/). Phone reachability requires an explicit LAN or NetBird bind; this is separate from the panel defect.

Acceptance criteria

• Only the Remote command panel occupies the full terminal viewport, including the composer/footer space.
• QR modules and quiet zone retain explicit black-on-white colours in the final frame, in light and dark themes.
• A full pairing QR is visible simultaneously when the terminal has enough rows; smaller terminals remain scrollable and show manual pairing details.
• Mouse selection matches the rendered Remote body; closing restores the usual UI.
• Other command panels retain their bounded layout.
• cargo fmt --all -- --check, cargo check --workspace --tests, and cargo test --workspace pass; record terminal verification limits.