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/math.ts

@@ -11,6 +11,24 @@ export function randomSeed(): number {
   return Math.floor(Math.random() * 2 ** 31);
 }
 
+/**
+ * Calculates the intersection point of two line segments.
+ *
+ * @param sp - Start point of the first line segment [x, y]
+ * @param ep - End point of the first line segment [x, y]
+ * @param sp2 - Start point of the second line segment [x, y]
+ * @param ep2 - End point of the second line segment [x, y]
+ * @param infinite - If true, treats the lines as infinite lines rather than line segments
+ * @returns The intersection point [x, y] if the lines intersect, null if they are parallel or coincident
+ *
+ * @example
+ * const intersection = lineIntersects([0, 0], [2, 2], [0, 2], [2, 0]);
+ * // Returns [1, 1] - the intersection point of the two line segments
+ *
+ * @example
+ * const parallel = lineIntersects([0, 0], [2, 2], [0, 1], [2, 3], true);
+ * // Returns null - the lines are parallel
+ */
 export function lineIntersects(
   sp: IVec,
   ep: IVec,
@@ -45,10 +63,23 @@ export function lineIntersects(
   return null;
 }
 
+/**
+ * Finds the nearest point on a polygon to a given point.
+ *
+ * @param points - Array of points defining the polygon vertices [x, y][]
+ * @param point - The point to find the nearest point to [x, y]
+ * @returns The nearest point on the polygon to the given point
+ * @throws Error if points array is empty or has less than 2 points
+ */
 export function polygonNearestPoint(points: IVec[], point: IVec) {
   const len = points.length;
-  let rst: IVec;
-  let dis = Infinity;
+  if (len < 2) {
+    throw new Error('Polygon must have at least 2 points');
+  }
+
+  let rst: IVec = points[0]; // Initialize with first point as fallback
+  let dis = Vec.dist(points[0], point);
+
   for (let i = 0; i < len; i++) {
     const p = points[i];
     const p2 = points[(i + 1) % len];
@@ -59,7 +90,7 @@ export function polygonNearestPoint(points: IVec[], point: IVec) {
       rst = temp;
     }
   }
-  return rst!;
+  return rst;
 }
 
 export function polygonPointDistance(points: IVec[], point: IVec) {

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

blocksuite/framework/std/src/gfx/viewport.ts

@@ -214,7 +214,12 @@ export class Viewport {
    * This property is used to calculate the scale of the editor.
    */
   get viewScale() {
-    if (!this._shell || this._cachedOffsetWidth === null) return 1;
+    if (
+      !this._shell ||
+      this._cachedOffsetWidth === null ||
+      this._cachedOffsetWidth === 0
+    )
+      return 1;
     return this.boundingClientRect.width / this._cachedOffsetWidth;
   }