fix: replace text-secondary with text-muted-foreground in branching components

In Tailwind v4, text-secondary resolves to --color-secondary (#2e3140),
a dark surface color — NOT --color-text-secondary (#848b9b). This made
all secondary text invisible on dark backgrounds.

The correct class is text-muted-foreground which maps to #848b9b.
This matches the pattern used by existing FlowPilot components.

Also reverts the unnecessary index.css variable bump from prior commit.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/index.css

@@ -20,7 +20,7 @@
   /* ── Text colors ───────────────────────────────── */
   --color-text-heading: #f0f2f5;
   --color-text-primary: #e2e5eb;
-  --color-text-secondary: #a0a7b8;
+  --color-text-secondary: #848b9b;
   --color-text-muted: #4f5666;
   --color-text-rail-label: #e2e5eb;
 
@@ -54,7 +54,7 @@
   --color-secondary: #2e3140;
   --color-secondary-foreground: #e2e5eb;
   --color-muted: #2e3140;
-  --color-muted-foreground: #a0a7b8;
+  --color-muted-foreground: #848b9b;
   --color-accent-tw: #2e3140;
   --color-accent-foreground: #e2e5eb;
   --color-destructive: #f87171;