fix: validate coordinates before elementsFromPoint on Firefox mobile (#1992)

Firefox mobile drag events can provide non-finite coordinate values
(`undefined`, `NaN`, `Infinity`) which cause
`document.elementsFromPoint()` to throw: "Argument 1 is not a finite
floating-point value."

**Changes**
- Added `Number.isFinite()` validation in `findProjectIdAtPosition()`
before calling `elementsFromPoint()`
- Returns `null` when coordinates are invalid, gracefully aborting the
drop operation

```typescript
function findProjectIdAtPosition(mouseX: number, mouseY: number): number | null {
	// Validate coordinates are finite numbers (required by elementsFromPoint)
	if (!Number.isFinite(mouseX) || !Number.isFinite(mouseY)) {
		return null
	}

	const elementsUnderMouse = document.elementsFromPoint(mouseX, mouseY)
	// ... rest of implementation
}
```

The caller already handles `null` returns appropriately, treating it as
"no valid drop target found."

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: kolaente <13721712+kolaente@users.noreply.github.com>

frontend/src/composables/useTaskDragToProject.ts

@@ -10,6 +10,12 @@ import {success, error} from '@/message'
  * Searches through elements under the mouse and their parents for data-project-id attribute.
  */
 function findProjectIdAtPosition(mouseX: number, mouseY: number): number | null {
+	// Validate coordinates are finite numbers (required by elementsFromPoint)
+	// This prevents errors on Firefox mobile where touch events may provide invalid coordinates
+	if (!Number.isFinite(mouseX) || !Number.isFinite(mouseY)) {
+		return null
+	}
+
 	const elementsUnderMouse = document.elementsFromPoint(mouseX, mouseY)
 
 	for (const el of elementsUnderMouse) {