Correct IME position calculation with window padding (#4949)

This PR addresses two IME-related issues:

1. Fix IME position calculation with window padding, fixed
#4933

- Add padding offset to IME position calculation to correctly position
the IME window
   
- This ensures IME window appears at the right position when
`window-padding-x/window-padding-y` are set

2. Clear selection when IME input starts

- Previously, when using CJK IME with text selected, the IME window
would appear near the selection
   
   - Now it clears selection and shows IME window at cursor position
   
   - This matches the behavior of iTerm2 and Terminal.app


## Before


https://github.com/user-attachments/assets/cd5c7b55-2083-40ce-a528-9a98898a6498

## After


https://github.com/user-attachments/assets/9f2c17c6-e885-45f3-9ab1-a9c3858690ec

src/Surface.zig

@@ -1316,8 +1316,8 @@ pub fn imePoint(self: *const Surface) apprt.IMEPos {
     const content_scale = self.rt_surface.getContentScale() catch .{ .x = 1, .y = 1 };
 
     const x: f64 = x: {
-        // Simple x * cell width gives the top-left corner
-        var x: f64 = @floatFromInt(cursor.x * self.size.cell.width);
+        // Simple x * cell width gives the top-left corner, then add padding offset
+        var x: f64 = @floatFromInt(cursor.x * self.size.cell.width + self.size.padding.left);
 
         // We want the midpoint
         x += @as(f64, @floatFromInt(self.size.cell.width)) / 2;
@@ -1329,8 +1329,8 @@ pub fn imePoint(self: *const Surface) apprt.IMEPos {
     };
 
     const y: f64 = y: {
-        // Simple x * cell width gives the top-left corner
-        var y: f64 = @floatFromInt(cursor.y * self.size.cell.height);
+        // Simple y * cell height gives the top-left corner, then add padding offset
+        var y: f64 = @floatFromInt(cursor.y * self.size.cell.height + self.size.padding.top);
 
         // We want the bottom
         y += @floatFromInt(self.size.cell.height);
@@ -1591,6 +1591,15 @@ pub fn preeditCallback(self: *Surface, preedit_: ?[]const u8) !void {
     self.renderer_state.mutex.lock();
     defer self.renderer_state.mutex.unlock();
 
+    // We clear our selection when ANY OF:
+    // 1. We have an existing preedit
+    // 2. We have preedit text
+    if (self.renderer_state.preedit != null or
+        preedit_ != null)
+    {
+        self.setSelection(null) catch {};
+    }
+
     // We always clear our prior preedit
     if (self.renderer_state.preedit) |p| {
         self.alloc.free(p.codepoints);