fix(users): trim profile username params

[aiirvizionz]

Summary

• trim and lowercase public profile username route params before lookup
• reject whitespace-only usernames before database work
• update route tests to cover trimming and blank username handling

Tests

• node node_modules\vitest\vitest.mjs run --config $env:TEMP\qryptchat-vitest-minimal.config.mjs "src/app/api/users/by-username/[username]/route.test.js"
• git diff --check

Note: I used the minimal Vitest config already present locally because the repo's default Vitest setup imports @vitejs/plugin-react, which currently fails against the installed Vite package export map in this checkout.