fix: correct Gemini model name from gemini-3-flash to gemini-3-flash-preview

The Gemini API requires the -preview suffix for the Gemini 3 Flash model.
gemini-3-flash does not exist - only gemini-3-flash-preview is available.
This was causing 404 errors when users selected this model option.

Closes #831

Co-Authored-By: Glucksberg <markuscontasul@gmail.com>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ui/viewer/components/ContextSettingsModal.tsx

@@ -465,7 +465,7 @@ export function ContextSettingsModal({
                     >
                       <option value="gemini-2.5-flash-lite">gemini-2.5-flash-lite (10 RPM free)</option>
                       <option value="gemini-2.5-flash">gemini-2.5-flash (5 RPM free)</option>
-                      <option value="gemini-3-flash">gemini-3-flash (5 RPM free)</option>
+                      <option value="gemini-3-flash-preview">gemini-3-flash-preview (5 RPM free)</option>
                     </select>
                   </FormField>
                   <div className="toggle-group" style={{ marginTop: '8px' }}>