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/routes/ralph-routes.ts

@@ -308,7 +308,7 @@ export function registerRalphRoutes(
       useMux: true,
       mode: 'claude',
       niceConfig,
-      model: rlModelConfig?.defaultModel,
+      model: rlModelConfig?.defaultModel || undefined,
       claudeMode: rlClaudeModeConfig.claudeMode,
       allowedTools: rlClaudeModeConfig.allowedTools,
     });

src/web/routes/session-routes.ts

@@ -186,7 +186,11 @@ export function registerSessionRoutes(
     const modelConfig = await ctx.getModelConfig();
     const mode = body.mode || 'claude';
     const model =
-      mode === 'opencode' ? body.openCodeConfig?.model : mode !== 'shell' ? modelConfig?.defaultModel : undefined;
+      mode === 'opencode'
+        ? body.openCodeConfig?.model
+        : mode !== 'shell'
+          ? modelConfig?.defaultModel || undefined
+          : undefined;
     const claudeModeConfig = await ctx.getClaudeModeConfig();
     const session = new Session({
       workingDir,
@@ -766,7 +770,11 @@ export function registerSessionRoutes(
     const niceConfig = await ctx.getGlobalNiceConfig();
     const qsModelConfig = await ctx.getModelConfig();
     const qsModel =
-      mode === 'opencode' ? openCodeConfig?.model : mode !== 'shell' ? qsModelConfig?.defaultModel : undefined;
+      mode === 'opencode'
+        ? openCodeConfig?.model
+        : mode !== 'shell'
+          ? qsModelConfig?.defaultModel || undefined
+          : undefined;
     const qsClaudeModeConfig = await ctx.getClaudeModeConfig();
     const session = new Session({
       workingDir: casePath,