Add model selector to plan messages proceed button

MODEL_SELECTOR_FOR_PLAN_MESSAGES.md

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+# Model Selector for Plan Messages Implementation
+
+## Summary
+When the "Proceed with Build" button appears in plan messages, a model selector dropdown is now displayed above it, allowing users to choose the AI model they want to use for the build process.
+
+## Changes Made
+
+### 1. New Helper Function: `createInlineModelSelector()`
+**Location:** `chat/public/builder.js` (lines ~1899-1975)
+
+Created a new helper function that generates an inline model selector specifically for plan messages:
+- Creates a labeled select dropdown with the title "Select Model for Build:"
+- Populates the dropdown with all available models from `state.models`
+- Shows model labels, names, and multiplier indicators (e.g., "2x")
+- Pre-selects the currently selected model from `state.selectedModelId`
+- Handles model selection changes and updates:
+  - The inline selector's value
+  - The main model selector in the composer area
+  - The state's selectedModelId
+  - Marks the change as user-initiated to prevent server overwrites
+
+### 2. Modified "Proceed with Build" Button Section
+**Location:** `chat/public/builder.js` (lines ~2174-2193)
+
+Updated the code where the "Proceed with Build" button is created:
+- Creates a container div (`buildActionsContainer`) to hold both the model selector and the button
+- Adds the inline model selector above the button
+- Maintains the existing "Proceed with Build" button functionality
+- Uses flexbox layout with proper spacing (gap: 12px, margin-top: 16px)
+
+## How It Works
+
+1. **Plan Message Displayed**: When an AI plan message is received and ready for building
+2. **Model Selector Created**: The `createInlineModelSelector()` function is called to generate the dropdown
+3. **User Selection**: User can select a different model for the build if desired
+4. **State Sync**: Selection changes are synchronized with the main model selector
+5. **Build Process**: When "Proceed with Build" is clicked, the selected model is used for the build
+
+## Benefits
+
+- **Better UX**: Users can now choose their model right where they need it (in the plan message)
+- **Flexibility**: No need to go back to the composer area to change models before proceeding
+- **Clear Context**: Model selector is clearly labeled as "Select Model for Build:"
+- **State Consistency**: Model selection is synchronized across all UI elements
+- **Prevents Conflicts**: User model selections are protected from server overwrites
+
+## Technical Details
+
+### Styling
+The inline model selector uses:
+- Consistent styling with the rest of the UI
+- Custom SVG dropdown arrow
+- Proper padding and borders matching the design system
+- Full-width layout for easy selection on all devices
+
+### State Management
+- Updates `state.selectedModelId` when changed
+- Calls `markUserModelChange()` to prevent server overwrites
+- Syncs with the main model selector in the composer
+- Preserves existing state management patterns
+
+### Integration
+- Works with the existing `updateModelSelectDisplay()` function
+- Compatible with `markUserModelChange()` for state protection
+- Integrates seamlessly with the current build flow
+- No changes to backend or API required
+
+## Testing
+- JavaScript syntax check: ✓ Passed
+- Function definition: ✓ Present at line 1900
+- Function call: ✓ Present at line 2181
+- No breaking changes to existing functionality