feat: support "Default (CLI default)" option for model selection

Allow users to leave the default model unset, so sessions use whatever
the Claude CLI defaults to rather than forcing a specific model.

- Add empty-value "Default (CLI default)" option to the model dropdown
- Treat empty string as undefined when passing model to Session
- Apply consistently across session creation, quick-start, and Ralph

src/web/public/index.html

@@ -1085,6 +1085,7 @@
             <div class="form-row">
               <label>Default Model</label>
               <select id="appSettingsDefaultModel" class="form-select">
+                <option value="">Default (CLI default)</option>
                 <option value="opus">Opus (Most capable)</option>
                 <option value="sonnet">Sonnet (Balanced)</option>
                 <option value="haiku">Haiku (Fast & cheap)</option>