fix(conversations): reject blank join ids

[aiirvizionz]

Summary

• Trim join conversationId values before validation.
• Reject whitespace-only ids before user lookup or SSE room join.
• Add regression coverage for blank ids and trimmed valid ids.

Validation

• git diff --check
• node node_modules\vitest\vitest.mjs run --config %TEMP%\qryptchat-vitest-minimal.config.mjs "src/app/api/conversations/join/route.test.js" (2 tests passed)

I used the same temporary minimal Vitest config needed locally because the repo's default Vitest config currently fails to load through the installed Vite/plugin combination; no dependency files were modified.