fix: pre-landing and adversarial review fixes

- landing.css: hardcode --lp-btn to #60a5fa (lesson 104 — no var(--color-*) in landing.css)
- ScriptBuilderInput: suggestion chips now correctly disabled during generation
- ChatSidebar: wrapper onClick no longer fires onSelect while in confirming state
- SessionHistoryPage: fix loadMoreAiSessions race condition with generation counter;
  flow session tab auto-activates when URL params target flow session filters

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/styles/landing.css

@@ -19,7 +19,7 @@
   --lp-accent: #60a5fa;
   --lp-accent-soft: rgba(96, 165, 250, 0.10);
   --lp-accent-text: #93bbfc;
-  --lp-btn: var(--color-accent, #f97316);
+  --lp-btn: #60a5fa;
   --lp-success: #34d399;
   --lp-danger: #f87171;
   --lp-warning: #fbbf24;