fix: PR review nits — response shape, landing copy, form reset

- /llm/generate's "model is downloading" branch was raising HTTPException(202, detail={...}), which wraps the payload in {"detail": ...} and forces clients to parse a success status as if it were an error. Switched to JSONResponse so the payload sits at the top level.
- The landing page's "Language Models" card advertised "Qwen 3.5" with sizes 4B/2B/0.8B; we ship Qwen3 at 0.6B/1.7B/4B. Aligned to what's actually in the binary.
- ProfileForm's discard-draft button reset the form without touching `personality` or `avatarFile`, so stale persona text and an attached avatar would survive the discard. The other three resets in the file already include both fields — this brings the discard path in line.

app/src/components/VoiceProfiles/ProfileForm.tsx

@@ -828,8 +828,10 @@ export function ProfileForm() {
                       name: '',
                       description: '',
                       language: 'en',
+                      personality: '',
                       sampleFile: undefined,
                       referenceText: '',
+                      avatarFile: undefined,
                     });
                     setSampleMode('record');
                   }}