🐞 Default tab size is incorrect (set to 4 but is only 3), changing it does not work

### Description

The default tab size is set to 4 in settings, but it is actually 3 in the editor window when counted against spaces. Changing the tab size in settings does not change the editor. A restart is also not honouring the changed tab size setting.

### To Reproduce

1. Go to settings->Text Editing
2. Default tab size is set to 4 spaces and Prefer Indent is set to Tabs
3. Close Settings.
4. Type 4 spaces in the editor
5. Type newline and a tab
6. Notice that the tab only equals 3 spaces, not 4.

### Expected Behavior

Tab size should be equal to the number of spaces set in settings, by default: 4 spaces not 3.

### Version Information

CodeEdit: 0.3.6 (47)
macOS: 15.7.1
Xcode: Version 26.1.1 (17B100)

### Additional Context

_No response_

### Screenshots

Changing tab size to some other value (here: 8) does not change the editor (also not after restart of CodeEdit):

從 Text Editing 設定開始，追蹤 tab-size 值如何傳遞到編輯器檢視；issue 中未指定特定檔案或測試。重現預設值和變更大小的情況，包括重新啟動後的情況；當編輯器一致地使用設定的空格數時，即可視為 issue 完成。

由索引模型根據 Issue 內容生成。