Document index.css consistently

[ZayanKhan-12]

Fixes #78.

index.css had five comments across 274 lines, written in two different styles (/** x */ and /** x **/), and they named sections without saying what the rules were for. Several blocks that are easy to get wrong carried nothing at all — which is what @nikhilpatil123 reported.

What the comments now say

Rather than restating the declarations, each block records what it styles, where its markup comes from, and what it is coupled to. The couplings are the reason most of this is worth writing down:

• The sidebar width is four values that must change together — #sidebar width 250px, #wrapper.toggled margin-left -250px, #toggle left 250px, and #contents margin-left 270px. Changing one alone misaligns the layout.
• The collapse transition is duplicated across three selectors (#wrapper, #sidebar, #toggle) and has to stay in sync for them to animate together.
• Two classes are applied from outside this file. index.js toggles toggled on #wrapper; IndexView.renderNav sets active on the nav group containing the current page. Neither is discoverable from the stylesheet.

The file header records something that is not obvious and changes how you read the whole sheet: IndexView loads this after bootstrap.min.css, so most rules exist to override a Bootstrap default. A rule that looks redundant usually is not — deleting it restores the Bootstrap appearance rather than changing nothing.

I also noted the selectors that appear nowhere in this repository's templates, so they don't read as dead code. .popover-content is generated by Bootstrap's own JavaScript, and .tabs-right / .tabs-below complete a set that services built on Twitter Server can use, of which StackRegistryView renders only .tabs-left.

Grounded in IndexView.scala, index.js and StackRegistryView.scala rather than inferred from selector names, so the comments describe what the code actually does.

Second commit: a bug found while documenting it

The spinner rule declared -animation. A vendor prefix has to name a vendor, so -animation matched nothing, browsers discarded the declaration, and only the -webkit- rule applied — the refresh spinner turned in Chrome and Safari and sat motionless in Firefox. The @keyframes spin rule it referenced was already present and, until now, unreachable.

Confirmed by parsing the declaration with a CSS engine:

declarations the parser kept:  -webkit-animation, animation
  -animation kept?         NO  - dropped as an unknown property

It is a one-word change and the only behaviour change in this PR, so it is a separate commit — drop it if you would rather keep this documentation-only, and I will open it on its own.

Verification

The documentation commit changes no behaviour, and I checked that rather than asserting it: stripping every comment from the file before and after leaves byte-identical CSS.

CSS with all comments stripped is identical: True
comment blocks  before: 5   after: 15

The stylesheet parses without errors (50 rules and at-rules) both before and after.

I could not run sbt clean test — there is no JVM on this machine, and CI's build also needs twitter/dodo-build to build the dependency chain. index.css is served as a static resource from /admin/files/css/, so nothing compiles against it, but the Scala suite is unrun on my side and needs CI to confirm.

I have not added a CLAUDE.md. Writing one that is actually useful means understanding the sbt/Bazel build and the Scala cross-version setup, and I could not build the project here; a file guessing at that would be worse than none.

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