A fixed-grid terminal (observe, mirrored Windows pane) cannot be panned on a small screen #230

What happens

A terminal whose grid is not the browser's own cannot be reached past the edge of a small screen. .pane-terminal has overflow: hidden, fitting is off for such a grid, and the touch handlers take vertical swipes for herdr's scrollback, so the columns to the right and the rows below the viewport (often the prompt) stay out of reach.

It affects two cases:

• an observe connection, which adopts the operator's grid (existing behaviour);
• a mirrored pane on a PC whose herdr cannot attach a terminal (Windows, feat(machines): mirror a pane's screen into the terminal lens where herdr cannot attach #228), where every client adopts the pane's own grid (pane-geometry with fixed: true).

Reproduce

1. bun run build, then UI_EVIDENCE_DIR=/tmp/shots bun scripts/windows-lens-browser-qa.ts.
2. Open windows-mirror-phone.png: a 120x40 grid on a 390 px wide screen shows its left part only. A longer line or a full screen puts the prompt outside the viewport.

Ideas

• Let the terminal host pan horizontally (overflow-x: auto, touch-action: pan-x) while vertical swipes keep scrolling herdr's history.
• Anchor a fixed grid to its bottom rows so the prompt is what stays visible.
• A pinch or a "fit width" toggle that scales the font down for a fixed grid.

Found by an adversarial review of #227 and #228. The chat lens, the terminal input line and the key bar still work on such a pane.