refactor: drive theme colors through CSS custom properties

Phase 2 (behavior-preserving):
- Add :root color tokens to app.css mirroring theme.ts cssVars
- Replace solid theme hex literals in SkillTree.svelte <style> with var(--..):
  #ffd700 -> --color-gold (x17), #a8a8b8 -> --text-muted (x8),
  #1a1a2e -> --panel-from, #0f0f1a -> --panel-to, #f9a825 -> --color-skills
- body background now var(--bg-space)

Values are identical to the literals; every referenced var has a :root
definition. rgba() shadows and one-off grays/badge colors left as-is.
check/lint/build all clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app.css

@@ -1,3 +1,19 @@
+/*
+ * Theme tokens — single source of truth for colours is src/lib/theme.ts.
+ * These custom properties mirror its `cssVars` map; keep the two in sync.
+ */
+:root {
+	--color-gold: #ffd700;
+	--color-experience: #4ecdc4;
+	--color-projects: #ff6b6b;
+	--color-skills: #f9a825;
+	--color-education: #dda0dd;
+	--bg-space: #05050a;
+	--panel-from: #1a1a2e;
+	--panel-to: #0f0f1a;
+	--text-muted: #a8a8b8;
+}
+
 *,
 *::before,
 *::after {
@@ -13,5 +29,5 @@ body {
 	margin: 0;
 	padding: 0;
 	overflow: hidden;
-	background: #05050a;
+	background: var(--bg-space);
 }