view: small state and interaction bugs on the sheet #2075

Found in the review of #2057, which adds the experimental view. It does not block that PR; #2059 fixed the findings that did. Paths refer to #2059's head.

Summary

Small state and interaction bugs on the view page, mostly in web/view/src/main.ts:

• Layout while hidden. In the "code" layout, the canvas is laid out while it is display: none. In overview mode the scale is 0/0, which is NaN, and showing the canvas again at the same width does not fix it. Mark the layout dirty while hidden, lay out again when shown, and guard the division.
• Reset races. The reset button does not wait for an edit in progress. pristineModels in web/view/src/githubHost.ts is set only after the background projection, and a failed load can leave the route and the project out of step. Discard stale results with a generation counter in the host.
• Selection cannot be cleared. Neither a click on empty canvas nor Escape clears it. Found by reading the code and the Pointer Events spec; not tried in a browser.
• Engine load failure sticks. loading ??= in web/view/src/engine.ts keeps a rejected promise, so the engine is never retried. The error tells the user to "run pnpm run engine", a developer instruction.
• "evaluating…" stays on screen when the workspace changes during an evaluation.
• A click inside a cell's input toggles trace mode.
• pendingPins survives open() and pins panes on the next sheet.