About's image fills the dialog on a Retina screen

[a-team-app]

Context

On a Retina screen in iTerm2 the About dialog's image is drawn at half width and half height, leaving the rest of the space reserved for it blank. iTerm2 reports the cell size in points plus a scale factor, and we ignore the scale. Reading it makes the image fill its space on every display. Pitch: #301.

Acceptance criteria

• In iTerm2 on a Retina screen, About (mnemonic tui) shows the artwork filling the area set aside for it, with no blank band between the image and the version line.
• On a non-Retina monitor in iTerm2 the About dialog looks exactly as it does today.
• In WezTerm and other sixel terminals the About dialog looks exactly as it does today.
• A terminal that sends a garbled cell size reply still opens About promptly, with a sensibly sized image or the text fallback.

Tests

• The iTerm2 cell size parser against the documented example ReportCellSize=17.50;8.00;2.0 → 16×35.
• The two-field form (older iTerm2) → scale 1.0, today's result.
• Non-integer scale (1.5) multiplied and rounded.
• Scale clamped silently to a sane range; non-positive or unparseable sizes still rejected so the other query or the default wins.
• AGENTS.md sixel note: iTerm2 reports points everywhere (XTWINOPS replies included), and only ReportCellSize carries the scale.

Out of scope

• Changing which query wins or the order they're sent; replacing SixelProbe with TG's detector.
• Any change to the artwork, image fitting, the sixel encoder, the encode cache or the ASCII fallback.
• The dark band at the image's bottom edge.
• Re-detecting on monitor or font changes mid-session.
• A scale setting.
• The diagnostics row (the next task).

[a-team-app]

Draft PR #346 is up.