Remove invalid CSS and the dead btnform block from forms.scss

[amjjbonvin]

Split out of #791 on review feedback — this is unrelated to that PR's CSS
pipeline fix and deserves its own review. It cherry-picked onto master cleanly,
confirming the two are independent.

Two changes, both to _sass/forms.scss

1. Three invalid declarations deleted

The file carried three declarations of the form margin-bottom: 5px / 2. Sass
never evaluated these — two literals with no parentheses emit the slash
verbatim, so the compiled CSS shipped margin-bottom:5px/2, which is invalid and
silently discarded by browsers.

They are deleted rather than converted to math.div(), because converting would
newly apply margins that have never been in effect. Nothing designed for the
values involved: 2.5px, 1.25px and 1.25px are artifacts of hardcoding numbers into
what was presumably variable arithmetic upstream.

Reach of each, measured against the built site:

Declaration	Reach
btnform p	dead — see below
label	dead — the site has no <label> element at all, nor any <textarea> or <select>
input, textarea, select	47 <input> elements, all on one legacy page (/software/haddock2.2/start_new/)

For the third, deleting preserves current rendering exactly: the invalid value
already resolved to no margin, so removing it changes nothing.

2. The dead btnform { ... } block deleted

Line 3 read btnform { — a bare element selector for <btnform>, which is not an
HTML element and appears zero times in the site's markup. That made 30 lines of
form styling dead code, including the first invalid declaration above.

Renaming it to form was considered and rejected. On the one page that has
forms, every nested rule would still match nothing — no fieldset, legend, p
or ul elements inside those forms, and all four <br> sit before the first
<form> tag. Renaming would activate only a 5px margin on three legacy forms,
while leaving form br { display: none } primed to silently hide line breaks in
any form added later.

Also drops the unused @use "sass:math" — the file uses only color.*. It was
left behind by a reverted change in #789.

Verification

• No invalid px/N values remain in the compiled CSS
• 0 HTML pages changed
• The only selectors removed are the six btnform* ones, all confirmed unmatched
• label and input,textarea,select keep every other declaration:

- label{display:block;margin-bottom:10px/8;font-weight:bold;color:#333332;cursor:pointer}
+ label{display:block;font-weight:bold;color:#333332;cursor:pointer}

Note on ordering: master currently serves the committed main_pretty.css, so
this change has zero live effect until #791 lands — verified, main_pretty.css
builds byte-identical here. The two PRs are safe to merge in either order.

Not addressed here

The same rule block sets width: 100% on inputs, which overrides the size=3 /
size=60 attributes on that legacy HADDOCK 2.2 form. Left alone — it is
long-standing behaviour and a separate question.

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