Add Tether (USDT) to ecosystem listing

[Copilot]

Adds Tether USDT stablecoin to the Base ecosystem directory.

Changes

• Metadata: Created apps/web/src/data/ecosystem/tether/metadata.json with defi/stablecoin categorization
• Ecosystem entry: Added to ecosystem.json at alphabetical position (index 502, between Test Project and Thales)
• Logo: Added tether.svg to partners directory (1.2KB SVG, green #50AF95 theme)

Entry Details

{
  "name": "Tether",
  "category": "defi",
  "subcategory": "stablecoin",
  "description": "USDT is a stablecoin pegged to the US dollar, providing stability and liquidity across blockchain networks.",
  "url": "https://tether.to",
  "imageUrl": "/images/partners/tether.svg"
}

Total ecosystem entries: 598 → 599

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