fix: customer-facing cleanup — remove internal branding, print→logging, dynamic dates, sandbox note, next steps

- main.py: neutral API description (remove 'sales demo')
- base.html: 'Open Banking Demo' nav, neutral footer
- demo.py: /debug-session gated behind DEMO_MODE, all print() → logging,
  webhook receiver has C# signature verification example + Tink docs link,
  removed duplicate import
- demo_data.py: all hardcoded 2026 dates replaced with dynamic date helpers
- step2.html: external_user_id terminology (remove tink_external_ref)
- step.html: sandbox note on Step 3 (anonymous vs production flow)
- step6.html: 'Next Steps' section for C#/.NET implementation

src/main.py

@@ -13,7 +13,7 @@ settings = get_settings()
 
 app = FastAPI(
     title="Tink API Demo",
-    description="MoneyCapp × Tink — sales demo showing v2 API endpoints",
+    description="Tink Open Banking API — integration walkthrough covering auth, users, accounts (v2), transactions (v2) and webhooks.",
     version="1.0.0",
 )