feat(site-nav): add sticky /sunset announcement bar to the nav header

[AnishSarkar22]

Description

Motivation and Context

FIX #

Screenshots

API Changes

• This PR includes API changes

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactoring
• Documentation
• Dependency/Build system
• Breaking change
• Other (specify):

Testing Performed

• Tested locally
• Manual/QA verification

Checklist

• Follows project coding standards and conventions
• Documentation updated as needed
• Dependencies updated as needed
• No lint/build errors or new warnings
• All relevant tests are passing

High-level PR Summary

This PR adds a sticky announcement banner to the top of the site navigation header to inform visitors that SurfSense is transitioning to a local app, with a link directing them to the /sunset announcement page. The banner can be toggled via an environment variable (NEXT_PUBLIC_TOP_ANNOUNCEMENT_ENABLED), supports dismissal via localStorage with versioning to allow future announcements to resurface, and is designed to stick together with the existing navigation header as a single unit to avoid stacking issues.

⏱️ Estimated Review Time: 5-15 minutes

💡 Review Order Suggestion

Order	File Path
1	surfsense_web/.env.example
2	surfsense_web/lib/env-config.ts
3	surfsense_web/components/ui/icons.tsx
4	surfsense_web/components/homepage/top-announcement-bar.tsx
5	surfsense_web/components/site/site-nav.tsx
6	surfsense_web/components/site/site-shell.tsx

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