refactor(decnet_web/css): promote hardcoded matrix/warn/crit colours to tokens

37 bare rgba(0, 255, 65, ...) literals across 10 component CSS
files were forcing matrix-green to bleed into light mode no matter
what data-theme=light overrode in :root. They're now mapped onto
existing tokens by alpha bucket (0.025-0.05 -> --matrix-tint-5,
0.08-0.10 -> --matrix-tint-10, 0.18-0.30 -> --matrix-tint-30,
0.4 -> --fg-4, 0.5-0.6 -> --fg-3, 0.7-0.8 -> --fg-2).

Adds --warn (#e0a040), --amber (alias of --warn), --crit
(#e74c3c), and their tint-10 variants to :root, with
ink-friendly light-mode overrides. Sweeps bare #ffaa00 / #e0a040
/ #f59e0b / #ff4d4d / #e74c3c usages in the same files onto the
new tokens.

Files with var(--token, #fallback) patterns left alone — those
were already token-driven and the fallbacks just provide safety.
Login.css and inline TSX hex left for the per-page sweep.

decnet_web/src/components/Credentials.css

@@ -48,7 +48,7 @@
   border: none;
   border-right: 1px solid var(--border);
   background: transparent;
-  color: rgba(0, 255, 65, 0.6);
+  color: var(--fg-3);
   cursor: pointer;
   font-family: inherit;
 }