Fix rerun-web canvas size

[jprochazk]

What

The canvas size was the default of 300x150 because we didn't update the CSS in index.html after

• Use ResizeObserver instead of resize event emilk/egui#4536

Closes

• Web viewer canvas is tiny #6507

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
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[jleibs]

This looks much simpler. However, I don't have any context on the historical reasons for the previous complexity, which always makes me a bit nervous.

[emilk]

I don't have any context on the historical reasons for the previous complexity, which always makes me a bit nervous.

Context: the old CSS was me copy-pasting things that I don't understand

[jprochazk]

However, I don't have any context on the historical reasons for the previous complexity, which always makes me a bit nervous.

I believe the original CSS here was taken from the egui demo app, or its predecessor. All it did was move the canvas into place, and then some code in eframe took care of resizing the canvas to the equivalent of width: 100%; height: 100%. That code in eframe is gone now, so that the size of the canvas can be controlled fully through CSS like is being done in this PR.