feat: Initial security review, hardening, and analysis

[a0preetham]

This commit incorporates findings and changes from an initial security review:

1. Core Logic (src/index.ts):

   • I refactored nanoid() generation during initial ID creation to remove an unused and confusing duplicate call.
   • I added HTTP security headers (CSP, X-Content-Type-Options, X-Frame-Options, HSTS) to responses serving static assets (client.html and others via env.ASSETS.fetch). A baseline CSP is provided and may need further tuning.

2. Security Analysis & Recommendations (Markdown Files):

   • jwt_secret_best_practices.md: Best practices for managing the JWT_SECRET.
   • id_usage_analysis.md: Analysis of the id used in JWTs and URLs, confirming it represents a capability-based access token for a specific sync session/document.
   • devmode_cookie_security_recommendations.md: Recommendations for cookie security flags in DEVMODE.
   • tinybase_security_analysis.md:
     • Confirms data scoping per id is handled by the Durable Object architecture.
     • Recommends defining TinyBase schemas on the server-side for data integrity.
     • Critically highlights that TinyBase does not sanitize string content, and the frontend is responsible for preventing Stored XSS when rendering data.
   • frontend_vulnerability_scan.md: Reports low risk for common frontend vulnerabilities (XSS, CSRF, QR code injection via TinyBase) based on a scan of the Angular application, noting good use of built-in framework protections.

Next Steps for You:

• Review all generated markdown reports for detailed findings and recommendations.
• Implement TinyBase schemas in SyncObject as recommended.
• Thoroughly review frontend rendering logic to ensure data from TinyBase is always sanitized before display to prevent XSS, as per tinybase_security_analysis.md.
• Perform a dependency vulnerability check: Run npm audit (or equivalent) in both the root and packages/frontend directories to identify and address known vulnerabilities in dependencies.
• Review and potentially tighten the Content Security Policy (CSP) in src/index.ts based on application needs.