Remove the unused @supabase/supabase-js dependency

Nothing in the app imported it; a leftover from the original template.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jWbNNJLksC1QG8z8845FX

package.json

@@ -20,7 +20,6 @@
         "@streamdown/code": "^1.1.1",
         "@streamdown/math": "^1.0.2",
         "@streamdown/mermaid": "^1.0.2",
-        "@supabase/supabase-js": "^2.51.0",
         "ai": "^7",
         "class-variance-authority": "0.7.1",
         "clsx": "^2.1.1",