feat(website): Support HS merchant themes in embed workspace

[raunak23427]

This commit implements the theme handshake for the embedded Decision Engine workspace, resolving Issue #455.

Changes include:

1. embedBridge.ts: Created a setupEmbedBridge function to listen for de:theme-update messages from the parent window. It verifies the frame ID and forwards validated theme tokens.
2. theme.ts: Added an applyThemeTokens function that securely validates and maps incoming tokens as CSS Custom Properties to the document root (e.g., --color-brand-500). It strictly filters for expected prefixes and rejects dangerous values like URLs or JS execution.
3. main.tsx: Initialized the setupEmbedBridge listener on application mount to ensure themes are received early.
4. index.css: Predefined the default fallback hex values for --color-brand-* variables.
5. tailwind.config.ts: Updated the Tailwind theme configuration to consume the dynamic CSS variables instead of hard-coded hex colors.

[raunak23427]

Implementation Notes for Reviewers: While reviewing Issue #236, I noticed that PR #284 recently migrated the GSM architecture to a high-performance in-memory CSV/S3 model. To respect this new architecture, I intentionally avoided introducing the PostgreSQL override layer originally suggested in the issue description.

Instead, I implemented the smallest, cleanest API extension possible:

Extended the internal GsmDecision enum with a Requeue state (mapped to RETRY_DIFFERENT_GATEWAY).
Added retry_decision, unified_code, and unified_message safely via .and_then() directly from the existing gsm_info cache hit.
Preserved the raw gsm_info object in the API response to ensure strict backward compatibility for any existing clients.
Handled the fail-closed fallback gracefully (defaulting to DO_NOT_RETRY if no rule matches).

[raunak23427]

"Hi @prajjwalkumar17 and @GauravRawat369! This PR finishes up the GSM retry mapping (Issue #236) on top of the new CSV architecture. It is fully green and passing all CI checks. I would really appreciate a review when you have a moment!"