Slice 2 Integration: Spawn Modal UI verification

## Overview
Integration checkpoint verifying all Slice 2 (UI) components work together.

## Background
Slice 2 delivers the complete spawn UI. This bead verifies:
- SpawnModal component
- App.js integration
- Toast notifications
- All user interactions

## Verification Checklist

### 1. Button Visibility
- [ ] '+ New Agent' button visible in header
- [ ] Button styled with active color scheme
- [ ] Button has hover state

### 2. Modal Opens
- [ ] Click button opens modal
- [ ] Modal has backdrop blur
- [ ] Modal animates in smoothly

### 3. Modal Context (Project Tab)
- [ ] Select a project in sidebar
- [ ] Open spawn modal
- [ ] Verify: NO project dropdown shown
- [ ] Verify: Shows 'Project: {name}' text
- [ ] Verify: Agent type selector visible

### 4. Modal Context (All Projects)
- [ ] Click 'All Projects' in sidebar
- [ ] Open spawn modal
- [ ] Verify: Project dropdown shown
- [ ] Verify: Dropdown says 'Loading...' initially
- [ ] Verify: Dropdown populates with projects

### 5. Agent Type Toggle
- [ ] Default is 'Claude' selected
- [ ] Click 'Codex' -> Codex selected
- [ ] Click 'Claude' -> Claude selected
- [ ] Only one can be selected

### 6. Modal Dismiss
- [ ] Press Escape -> modal closes
- [ ] Click outside modal -> modal closes
- [ ] Click Cancel button -> modal closes
- [ ] Modal animates out smoothly

### 7. Spawn (Project Tab)
- [ ] Select project in sidebar
- [ ] Open modal, select agent type
- [ ] Click Spawn
- [ ] Button shows 'Spawning...'
- [ ] Modal closes on success
- [ ] Success toast appears
- [ ] Agent appears in Zellij

### 8. Spawn (All Projects)
- [ ] On All Projects tab
- [ ] Open modal
- [ ] Select project from dropdown
- [ ] Select agent type
- [ ] Click Spawn
- [ ] Verify success flow

### 9. Error Handling
- [ ] Trigger rate limit error
- [ ] Verify error shows in modal
- [ ] Error toast appears

### 10. Loading State
- [ ] Spawn button disabled during request
- [ ] Cancel button disabled during request
- [ ] Cannot dismiss via Escape during request

## UI/UX Checklist
- [ ] Colors match dashboard theme
- [ ] Fonts consistent
- [ ] Spacing looks good
- [ ] No layout shifts
- [ ] Works in narrow viewport

## Acceptance Criteria Covered
- AC-1: Button in header
- AC-2: No project picker on project tab
- AC-3: Project picker on All Projects
- AC-6: No default project selection
- AC-7: Agent type selector
- AC-20, AC-21: Error and success toasts
- AC-25: Button disabled during spawn
- AC-31: Modal dismiss methods
- AC-32: Loading state for dropdown

## Success Criteria
All 10 verification steps pass. User can spawn agents from both project tab and All Projects views.

Verified: SpawnModal displays as dropdown under New Agent button. Screenshot confirms correct positioning, no blur overlay, project picker, agent type toggle, and proper styling. Implementation matches user request for dropdown instead of modal overlay.