refactor: remove duplicate TailwindCSS utility definitions from utilities.css (#1089)

* Initial plan

* Remove duplicate TailwindCSS utility definitions from utilities.css

Enable TailwindCSS v4 PostCSS integration via postcss.config.mjs and
@import "tailwindcss" in index.css. Remove ~377 lines of duplicate
utility class definitions (flexbox, spacing, typography, grid, borders,
shadows, sizing, overflow, aspect-ratio) that TailwindCSS now provides.

Retain all custom project utilities (widget-content, cyber-border,
neon-text, animations, security level colors), dark mode overrides,
and CSS custom property mappings. Consolidate identical-value dark
mode overrides for reduced line count.

File size: 946 → 569 lines (39.9% reduction, under 570 target)

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* Add clarifying comments for consolidated dark mode overrides

Address code review feedback: add comments explaining why different
gray levels share dark mode values and why both light/dark selectors
are needed for CSS custom property overrides.

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* Remove conflicting gap-sm/md/lg definitions from utilities.css

These custom gap values (0.5rem/0.75rem/1rem) didn't match the design
token spacing scale (sm=6px, md=8px, lg=16px). Now that Tailwind is
enabled, it generates gap-sm/md/lg from the configured spacing scale
in tailwind.config.ts, ensuring consistency with design tokens.

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---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: pethers <1726836+pethers@users.noreply.github.com>

Author: Copilot

postcss.config.mjs

@@ -0,0 +1,5 @@
+export default {
+  plugins: {
+    "@tailwindcss/postcss": {},
+  },
+};

src/index.css

@@ -1,6 +1,9 @@
 /* Main index.css - import all other CSS files here */
 /* Check for any loose variable declarations here too */
 
+/* TailwindCSS v4 — provides standard utility classes */
+@import "tailwindcss";
+
 /* Import other CSS files */
 @import "./styles/variables.css";
 @import "./styles/utilities.css";