fix: white background option for export + Firefox download

Fixes two issues reported in #165:

- Export always produced a black background, making prints with a
  white page wasteful/unreadable. Adds a Dark/White background choice
  in the export modal, threaded through both PNG and SVG export.
- Firefox refused the download because the programmatic anchor was not
  attached to the document. The anchor is now appended before click and
  removed after, which Firefox requires.

Closes #165

ha-relevant: yes

frontend/src/components/modals/ExportModal.tsx

@@ -2,7 +2,15 @@ import { useState } from 'react'
 import { Download, Loader2 } from 'lucide-react'
 import { Dialog, DialogContent, DialogHeader, DialogTitle, DialogFooter } from '@/components/ui/dialog'
 import { Button } from '@/components/ui/button'
-import { exportToPng, exportToSvg, EXPORT_QUALITY_OPTIONS, type ExportQuality, type ExportFormat } from '@/utils/export'
+import {
+  exportToPng,
+  exportToSvg,
+  EXPORT_QUALITY_OPTIONS,
+  EXPORT_BACKGROUND_OPTIONS,
+  type ExportQuality,
+  type ExportFormat,
+  type ExportBackground,
+} from '@/utils/export'
 
 interface ExportModalProps {
   open: boolean
@@ -13,6 +21,7 @@ interface ExportModalProps {
 export function ExportModal({ open, onClose, getElement }: ExportModalProps) {
   const [quality, setQuality] = useState<ExportQuality>('high')
   const [format, setFormat] = useState<ExportFormat>('png')
+  const [background, setBackground] = useState<ExportBackground>('dark')
   const [exporting, setExporting] = useState(false)
 
   const handleExport = async () => {
@@ -21,9 +30,9 @@ export function ExportModal({ open, onClose, getElement }: ExportModalProps) {
     setExporting(true)
     try {
       if (format === 'svg') {
-        await exportToSvg(el)
+        await exportToSvg(el, background)
       } else {
-        await exportToPng(el, quality)
+        await exportToPng(el, quality, background)
       }
       onClose()
     } finally {
@@ -70,6 +79,32 @@ export function ExportModal({ open, onClose, getElement }: ExportModalProps) {
           </button>
         </div>
 
+        <div className="space-y-1.5 pb-2">
+          <p className="text-xs font-medium text-muted-foreground">Background</p>
+          <div className="flex gap-2">
+            {EXPORT_BACKGROUND_OPTIONS.map((opt) => (
+              <button
+                key={opt.value}
+                type="button"
+                onClick={() => setBackground(opt.value)}
+                className={[
+                  'flex-1 flex items-center gap-2 px-3 py-2 rounded-md border text-sm transition-colors',
+                  background === opt.value
+                    ? 'border-[#00d4ff] bg-[#00d4ff10] text-foreground'
+                    : 'border-border bg-[#0d1117] text-muted-foreground hover:border-muted-foreground',
+                ].join(' ')}
+              >
+                <span
+                  className="h-3.5 w-3.5 rounded-sm border border-border"
+                  style={{ background: opt.color }}
+                />
+                <span className="font-medium">{opt.label}</span>
+                <span className="text-xs opacity-70">{opt.hint}</span>
+              </button>
+            ))}
+          </div>
+        </div>
+
         <DialogFooter className="gap-2">
           <Button variant="ghost" onClick={onClose} disabled={exporting}>Cancel</Button>
           <Button

frontend/src/components/modals/__tests__/ExportModal.test.tsx

@@ -12,6 +12,10 @@ vi.mock('@/utils/export', () => ({
     { value: 'high',     label: 'High',     pixelRatio: 2, hint: '2× — recommended' },
     { value: 'ultra',    label: 'Ultra',    pixelRatio: 4, hint: '4× — print quality, large file' },
   ],
+  EXPORT_BACKGROUND_OPTIONS: [
+    { value: 'dark',  label: 'Dark',  color: '#0d1117', hint: 'screen / docs' },
+    { value: 'white', label: 'White', color: '#ffffff', hint: 'printing' },
+  ],
 }))
 
 const el = document.createElement('div')
@@ -49,7 +53,7 @@ describe('ExportModal', () => {
     render(<ExportModal open onClose={onClose} getElement={getElement} />)
     fireEvent.click(screen.getByText('Standard').closest('button')!)
     fireEvent.click(screen.getByRole('button', { name: /download/i }))
-    await waitFor(() => expect(mockExportToPng).toHaveBeenCalledWith(el, 'standard'))
+    await waitFor(() => expect(mockExportToPng).toHaveBeenCalledWith(el, 'standard', 'dark'))
   })
 
   it('renders an SVG option under the quality options', () => {
@@ -62,7 +66,7 @@ describe('ExportModal', () => {
     fireEvent.click(screen.getByText('SVG').closest('button')!)
     expect(screen.getByText('SVG').closest('button')!.className).toContain('border-[#00d4ff]')
     fireEvent.click(screen.getByRole('button', { name: /download/i }))
-    await waitFor(() => expect(mockExportToSvg).toHaveBeenCalledWith(el))
+    await waitFor(() => expect(mockExportToSvg).toHaveBeenCalledWith(el, 'dark'))
     expect(mockExportToPng).not.toHaveBeenCalled()
   })
 
@@ -74,6 +78,28 @@ describe('ExportModal', () => {
     expect(screen.getByText('SVG').closest('button')!.className).not.toContain('border-[#00d4ff]')
   })
 
+  it('renders dark and white background options, dark selected by default', () => {
+    render(<ExportModal open onClose={onClose} getElement={getElement} />)
+    expect(screen.getByText('Dark')).toBeInTheDocument()
+    expect(screen.getByText('White')).toBeInTheDocument()
+    expect(screen.getByText('Dark').closest('button')!.className).toContain('border-[#00d4ff]')
+  })
+
+  it('exports with white background when White is selected (printing)', async () => {
+    render(<ExportModal open onClose={onClose} getElement={getElement} />)
+    fireEvent.click(screen.getByText('White').closest('button')!)
+    fireEvent.click(screen.getByRole('button', { name: /download/i }))
+    await waitFor(() => expect(mockExportToPng).toHaveBeenCalledWith(el, 'high', 'white'))
+  })
+
+  it('applies the background choice to SVG export too', async () => {
+    render(<ExportModal open onClose={onClose} getElement={getElement} />)
+    fireEvent.click(screen.getByText('SVG').closest('button')!)
+    fireEvent.click(screen.getByText('White').closest('button')!)
+    fireEvent.click(screen.getByRole('button', { name: /download/i }))
+    await waitFor(() => expect(mockExportToSvg).toHaveBeenCalledWith(el, 'white'))
+  })
+
   it('closes after successful export', async () => {
     render(<ExportModal open onClose={onClose} getElement={getElement} />)
     fireEvent.click(screen.getByRole('button', { name: /download/i }))