Plan: Update the default VAT rate to 0 in adding/editing forms and ensure all VAT fields are formatted to 2 decimals throughout the app.

Changed:
* `index.php`: 
  - Changed the default `vat_rate` value to `0` in both PHP backend processing and JavaScript calculations.
  - Set the HTML input step for `vat_rate` to `0.01` with a default `value="0"`.
  - Updated all VAT percentage and VAT amount display formatting to use exactly 2 decimal places (`.toFixed(2)` in JavaScript and `number_format(..., 2)` in PHP) globally.

Notes:
* This change applies to item forms, shopping cart/POS, sales invoices, purchase invoices, and VAT reporting.
* Reminder: click Save in the editor to sync changes.

Next: Would you like any other form default values or formats adjusted?