The web renderers handle `Image` inconsistently with each other and with the spec

[andrewkolos]

The three web renderers each implement the basic catalog Image differently, leading to visibly different rendered output for the same A2UI input. They also diverge from the spec on the default fit value.

Renderer	Implementation	Default fit
React	Bare <img> (Image.tsx#L53)	'fill' (Image.tsx#L26) — matches spec
Lit	<img> inside an a2ui-image custom element (Image.ts#L25, Image.ts#L87-L93) with width: 100%; height: auto in shadow CSS (Image.ts#L38-L44)	'fill' inline (Image.ts#L84), but the wrapper's height: auto might make it visually irrelevant?
Angular	Bare <img> (image.component.ts#L37-L44) with max-width: 100%; height: auto in component CSS (image.component.ts#L47-L52)	'cover' (image.component.ts#L78) — diverges from spec
Spec		'fill' (basic_catalog.json#L62-L67)

One consequence of this can be seen in #1297, which is an issue that the React angular has that the Lit and Angular ones don't.

Ideally, we want rendering behavior to be consistent and predictable across renderers, especially ones residing in the same repo and using the same underlying rendering tech (CSS). This is a larger problem that would require a great deal more work to fix and prevent regression for, so I leave it beyond the scope of this issue and focus on Image here instead.

We should pick a target behavior for Image, document it, and align all three renderers and the spec on it.