Update branding and improve YouTube channel functionality

[mhaadiabu]

This pull request implements public YouTube channel lookup and import using the YouTube Data API v3, replacing mock data and extending the data model and backend logic to support public-only channel metadata. It also updates the UI, environment configuration, and documentation to reflect these changes, and improves the influencer persistence logic to support upserting by YouTube channel ID. Additionally, it introduces minor UI improvements and updates image configuration for profile images.

YouTube Public Lookup Integration:

• Added a comprehensive plan for implementing public YouTube channel lookup and import, outlining API usage, input normalization, data mapping, error handling, and delivery steps (PLAN.md).
• Updated documentation to describe the new YouTube public lookup flow, required environment variables, and supported input types in README.md. [1] [2]

Backend and Data Model Enhancements:

• Extended the influencer schema and backend logic to support additional YouTube fields (e.g., customUrl, profileImageUrl, description, subscribers, uploadsPlaylistId, topicCategories, country, channelCreatedAt, and metadata fields for source tracking and refresh timestamps), and added a new upsert mutation for YouTube influencers (convex/schema.ts, convex/influencers.ts). [1] [2] [3] [4] [5] [6]
• Added a new index on channelId for efficient upserts and lookups in the influencer table (convex/schema.ts).

Business Logic and Sorting:

• Updated the sorting logic for top influencers to fall back to totalViews if estimatedAnnualRevenue is unavailable, ensuring meaningful ranking for public YouTube imports (convex/analytics.ts).

Frontend and UI Improvements:

• Updated the sign-in page to use the GRA logo image instead of a text placeholder, and made minor style adjustments (src/app/(auth)/sign-in/page.tsx). [1] [2] [3] [4] [5]
• Updated Next.js image config to allow YouTube profile images from known hostnames (next.config.ts).

References:
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]