Add model selector to plan messages proceed button

IMPLEMENTATION_SUMMARY.txt

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+# Model Selector for Plan Messages - Implementation Complete
+
+## What Was Implemented
+
+When the "Proceed with Build" button appears in plan messages, a model selector dropdown is now displayed above it, allowing users to choose which AI model to use for the build process.
+
+## Changes Made
+
+### 1. New Function: `createInlineModelSelector()`
+Location: chat/public/builder.js (line 1900-1975)
+
+This helper function:
+- Creates a container div with the class 'inline-model-selector'
+- Adds a label "Select Model for Build:" above the dropdown
+- Creates a styled select dropdown element
+- Populates it with all available models from state.models
+- Shows model labels, names, and multipliers (e.g., "2x")
+- Pre-selects the currently chosen model
+- Handles user selection changes and syncs with the main model selector
+- Updates state.selectedModelId and calls markUserModelChange()
+
+### 2. Modified "Proceed with Build" Button Section
+Location: chat/public/builder.js (line 2174-2193)
+
+The proceed button section now:
+- Creates a container div (buildActionsContainer) using flexbox layout
+- Adds the inline model selector above the button
+- Adds the "Proceed with Build" button below the selector
+- Appends the entire container to the message body
+
+## Key Features
+
+ Model selector appears inline in plan messages
+ User can change model before proceeding with build
+ Selection syncs with main model selector in composer
+ State is properly managed and protected from server overwrites
+ Handles edge cases (no models available)
+ Matches existing UI styling and design system
+ JavaScript syntax validation passed
+ No breaking changes to existing functionality
+
+## How It Works
+
+1. User receives a plan message from AI
+2. The renderMessages function detects it's a plan message ready for building
+3. A buildActionsContainer is created with the inline model selector and proceed button
+4. User can select a different model from the dropdown if desired
+5. Selection changes update state and sync with main selector
+6. When user clicks "Proceed with Build", the selected model is used for the build
+
+## Technical Details
+
+- Uses flexbox for layout (gap: 12px, margin-top: 16px)
+- Select element styled with custom SVG dropdown arrow
+- Full-width design for easy selection on all devices
+- Integrates with existing state management functions
+- No backend changes required