fix(admin): use blue IAM mark as favicon - #1729

Summary

The admin app's favicon was missing — browsers showed no tab icon. The in index.html pointed at /vite.svg, which doesn't exist in the app's public/ directory. This PR points the favicon at the blue IAM brand mark so the tab icon matches the login/sidebar branding

Changes

• Copied the blue IAM mark (src/assets/icons/iam.svg, fill="#3E63DD") into public/iam.svg so Vite serves it as a static asset at /iam.svg.
• Updated index.html favicon reference from the non-existent /vite.svg to /iam.svg.

Technical Details

• The IAM mark previously existed only in src/assets/icons/ and was imported as a React component (iam.svg?react) for the login and sidebar — it wasn't available as a servable static file for use in index.html. Favicons referenced from index.html must resolve to a static file, so a copy in public/ is the conventional Vite approach.
• The original src/ copy is left untouched; the React imports in Sidebar/login/magiclink are unaffected. There's a public/logo.svg, but it's a plain shield with a near-black fill (#171717), not the blue brand mark, so it wasn't used.
• Change is fully isolated to the admin app: the SDK has no favicon/index.html references, and public/* is copied verbatim into dist/admin (embedded via embed.go) with no naming collision.

Test Plan

• Manual testing completed
• Build and type checking passes

SQL Safety (if your PR touches *_repository.go or goqu.*)

N/A