The New forge page asks for a name and a size, and nothing else #3095

From #2914 step 2 ("Make my forge asks only a name and a size").

Done looks like:

• [ ] src/web/pages/new-forge.ts shows the three sizes as a radio group with each size's CPU, memory and disk from src/spawn/sizes.ts, small checked, written as its own markup. The shared selectField in src/web/ui.ts is not changed (changing it broke test/ssh-site.test.ts twice in #2980).
• [ ] The chosen size reaches the plan; a test posts the form with medium and finds it in the plan.
• [ ] The page renders whole without JavaScript, and a 360px render shows every size's label and figures without hovering.

Files: src/web/pages/new-forge.ts, test/new-forge.test.ts.

Dropped: it repeats #2996, which already covers the New forge form's size choice. #2996 now waits for #3094.