fix(report): restore the centered plus icon in the image upload box

main.css already ships the reference plus icon (.preview .add:before/:after,
25px rounded bars centered via inset:0 + margin:auto). The duplicate
left/top:50% + translate version in modal-runtime.css over-constrained
the same pseudo elements (right/bottom:0 + margin:auto from main.css
still applied), pushing the bars to 75%/75% so only an L-shaped corner
was visible. Drop the duplicate and lock the single-source contract in
the report test.

tests/widget-system/report-ban-contract.test.cjs

@@ -195,6 +195,9 @@ test('report modal mirrors the reference theme: preset reasons, image evidence,
   const css = read('assets/css/frontend/modal-runtime.css')
   assert.match(css, /\.aksrt-report-modal:not\(\.aksrt-modal-runtime--bottom\) \.aksrt-modal-runtime__dialog/)
   assert.match(css, /600px/)
+  // 加号图标单一来源：main.css 的 .preview .add:before/:after（inset:0 + margin:auto），
+  // modal-runtime 不得重复定义（否则过约束冲突，加号偏移只剩“L”形）。
+  assert.ok(!/form-upload \.preview \.add::?(before|after)/.test(css), 'duplicated add plus rules must stay out of modal-runtime.css')
 })
 
 test('comments expose a report entry for other users when reporting is enabled', () => {