fix(editor): connector target position NaN (#11606)

Close [BS-3086](https://linear.app/affine-design/issue/BS-3086/frame里套frame，连一下connector，拖两下，白板损坏)

### What Changes
- Fixed `bound.toRelative` may be return `NaN` when `bound.w === 0 || bound.h ===0`
- Remove type assertions from `connector-manager.ts` for more type safety

blocksuite/framework/global/src/gfx/model/bound.ts

@@ -341,8 +341,15 @@ export class Bound implements IBound {
     return serializeXYWH(this.x, this.y, this.w, this.h);
   }
 
+  /**
+   * Convert a point to relative coordinates.
+   * @param point - The point to convert.
+   * @returns The normalized relative coordinates of the point.
+   */
   toRelative([x, y]: IVec): IVec {
-    return [(x - this.x) / this.w, (y - this.y) / this.h];
+    const normalizedX = this.w === 0 ? 0 : (x - this.x) / this.w;
+    const normalizedY = this.h === 0 ? 0 : (y - this.y) / this.h;
+    return [normalizedX, normalizedY];
   }
 
   toXYWH(): XYWH {

blocksuite/framework/global/src/gfx/model/vec.ts

@@ -565,6 +565,8 @@ export class Vec {
    * @param n
    * @param min
    */
+  static clampV(A: IVec, min: number, max?: number): IVec;
+
   static clampV(A: number[], min: number): number[];
 
   // eslint-disable-next-line @typescript-eslint/unified-signatures