fix: initialize file editor and remove broken individual file uploads

Changes:
- Add fileEditor.init() call in DOMContentLoaded to activate edit buttons
- Remove individual file upload inputs (projectInput, svgInput, cssInput, csvInput)
  that had CORS issues when loading project configurations
- Keep only the folder picker which works reliably
- Update UI to emphasize folder picker as the primary loading method
- Remove corresponding event handlers from engine.js
- Remove tests for individual file upload functionality

The folder picker loads all project files in one operation without CORS
issues, while individual file uploads failed when trying to load
referenced files (CSV, SVG, CSS) from the project.json.

All 56 tests passing.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test/integration.test.js

@@ -310,78 +310,7 @@ describe('Timeline Integration', () => {
     })
   })
 
-  describe('DOM Event Handling', () => {
-    it('should handle project file upload', async () => {
-      const config = createSampleProject()
-      const projectInput = document.createElement('input')
-      projectInput.id = 'projectInput'
-      document.body.appendChild(projectInput)
-
-      // Setup event handlers
-      window.setupEventHandlers()
-
-      // Mock file reading
-      const mockFile = new File([JSON.stringify(config)], 'project.json', { type: 'application/json' })
-      mockFile.text = vi.fn().mockResolvedValue(JSON.stringify(config))
-
-      // Mock the fetch calls that loadProjectConfigObject will make
-      mockFetch('/* test css */') // CSS
-      mockFetch(createSampleTemplate())
-      mockFetch(createSampleCSV())
-      mockPapaParse()
-
-      // Simulate file selection
-      Object.defineProperty(projectInput, 'files', {
-        value: [mockFile],
-        writable: false
-      })
-
-      // Trigger the event
-      const event = new Event('change')
-      projectInput.dispatchEvent(event)
-
-      // Wait for async operations
-      await new Promise(resolve => setTimeout(resolve, 0))
-
-      expect(document.getElementById('projectName').textContent).toBe('Test Project')
-    })
-
-    it('should handle CSV file upload', async () => {
-      const config = createSampleProject()
-      timelineEngine.config = config
-      timelineEngine.template = createSampleTemplate() // Need template for generation
-
-      const csvInput = document.createElement('input')
-      csvInput.id = 'csvInput'
-      document.body.appendChild(csvInput)
-
-      // Setup event handlers
-      window.setupEventHandlers()
-
-      const csvContent = createSampleCSV()
-      const mockFile = new File([csvContent], 'data.csv', { type: 'text/csv' })
-      mockFile.text = vi.fn().mockResolvedValue(csvContent)
-
-      global.Papa.parse.mockImplementation((text, options) => {
-        options.complete({
-          data: [{ ID: 'T-1', Title: 'Uploaded Task', Lane: 'Upload Lane', Due: '2025-01-15' }]
-        })
-      })
-
-      Object.defineProperty(csvInput, 'files', {
-        value: [mockFile],
-        writable: false
-      })
-
-      const event = new Event('change')
-      csvInput.dispatchEvent(event)
-
-      await new Promise(resolve => setTimeout(resolve, 0))
-
-      expect(timelineEngine.csvOverride).toBe(true)
-      expect(document.getElementById('viewer').innerHTML).toContain('Uploaded Task')
-    })
-  })
+  // DOM Event Handling tests removed - individual file uploads replaced with folder picker
 
   describe('SVG Export', () => {
     it('should hide IDs in external view for export', async () => {