chore(viewport): drop the compare-form scaffold that was never written

[adamjohnwright]

CompareFormComponent is the Angular CLI's generated placeholder:

<p>compare-form works!</p>

Eleven lines of TypeScript, an empty stylesheet, that template. In the tree since the initial commit, and rendered into every pathway browser page inside a panel that can never open — the panel is gated on dropdown() === 'compare', and nothing anywhere sets that state.

Why bother

It cost me twenty minutes today. Chasing whether the toolbar's Compare button worked, I found a component named compare-form, a dropdown state called 'compare', and no way to reach either — which reads like a broken feature rather than an unfinished one. The next person pays the same.

Comparing against a disease variant is the Compare button in the toolbar, and it works: it opens a menu of the disease variants and navigates to the one you pick. That is the whole feature; nothing is being removed from it.

Changes

• Delete compare-form/ (3 files).
• Remove the unreachable panel from viewport.component.html.
• Narrow dropdown from 'analysis' | 'compare' | null to 'analysis' | null, with a note saying why, so this does not get re-scaffolded.

Verified on beta

• Compare still lists "Defective Intrinsic Pathway for Apoptosis" and navigates.
• Analyze still opens its form (.dropdown.open).
• No cr-compare-form left in the DOM, no console errors.
• Full preflight green.