fix(web): add validation for website link fields

[guangshinhaha]

Summary

• If either URL or description is filled in a website link row, the other field becomes required — matching PG's cross-field validation behavior (closes [FE] Create 'View only' post - No validation check for website links #37)
• URL format validation triggers immediately on input — only http:// and https:// URLs are accepted
• Invalid fields show inline error messages with aria-invalid destructive styling
• Submit button is disabled when any link row has validation errors

⚠️ PGW cross-check needed

These validation rules were inferred from the PG screenshots in #37 — we're running in mock mode (TW_PG_MOCK=true) and haven't verified against PGW's actual FE/BE. Before merging, need to confirm:

1. Whether PGW requires both fields when either is filled (vs. allowing URL-only)
2. Whether PGW validates URL format client-side or only server-side
3. Whether PGW restricts to http(s):// or allows other protocols (e.g. tel:, mailto:)

Test plan

• Add a website link row → fill only URL → verify description shows "Description is required." error, submit disabled
• Add a website link row → fill only description → verify URL shows "URL is required." error, submit disabled
• Type an invalid URL (e.g. not-a-url) with a description → verify "Please enter a valid URL." error
• Fill both fields with valid https:// URL → verify no errors, submit enabled
• Leave a link row completely empty → verify no errors (empty rows are pruned)
• Cross-check rules against PGW's actual FE/BE codebase

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