[5/7] Size exports from the edit and scale bitrate with it

[dazzatronus]

Without output.size, exports came out at 1920×1080 whatever the edit's resolution and aspectRatio, at a fixed 5 Mbps. The size follows edit.size, the size the canvas is laid out at, and the bitrate scales with pixel count: 5 Mbps at 1080p, floored at 1 Mbps. The pre-flight browser check uses the same size and bitrate as the encode.

#	PR	Adds	Why separate
1	#224	exportBlob as the export engine	The new API
2	#225	Pre-flight issues; fail on placeholders	Routing depends on it
3	#226	Off-screen frames	Stops the page jumping
4	#227	Clip speed in frames and audio	Its own fix
5 (this PR)	#228	Size and bitrate from the edit	Its own fix
6	#229	Refuse or correct the remaining wrong exports	Found in the whole-stack review
7	#230	Release 2.25.0	Mechanical

Verify: npm run verify:ci; in a browser, animations.json at hd + 9:16 exports at 720×1280.