feat: Add "Add to quantity" and "Subtract from quantity" actions

[kaligrafy]

Add two new quantity adjustment actions that allow users to quickly increment or decrement the quantity of selected items by a specified amount, without needing to calculate the new total manually.

Changes:

• Add edit_qty_add and edit_qty_subtract actions in ActionManager
• Add Document::addQuantity() method with overflow checking
• Add actions to Edit > Quantity submenu
• Add actions to Quantity column context menu
• Add toolbar buttons (list-add/list-remove icons)
• Add actions to mobile ViewEditMenu

Also fixes a missing brace in desktopapplication.cpp that caused a build error.

Closes #1002

[kaligrafy]

Some screenshots:

[paramecie]

Or easier, one single interface "Add to qty" but allowing negative quantity like -3 which then subtracts.
You don't need 2 interfaces.

[paramecie]

Alternatively, we could have a more generic "Edit Qty" or "Calc Qty" (say current qty is 100):

• if number has no sign or symbol, then it simply updates the quantity (120 => changes to 120)
• if number is prefixed by "+" it adds (+33 => 133)
• if number is prefixed by "-" it subtracts (-18 => 82)

By extension:

• if number is prefixed by "*" it multiplies (*2 => 200)
• if number is prefixed by "/" it divides (if no modulo) (/4 => 25)

We'd ignore "*-2" or a symbol and a sign (vice versa), which probably is too complicated or rare to be of any usage.

[ZZJHONS]

The Quantity column already has a right click menu item to set quantity and it has buttons for +/- the only problem is that it defaults to the last used numbers but when selecting only one lot it could have the old value.

[paramecie]

What? I don't have any button + or - in the Set Qty from contextual menu. Just a single text field with the value. Latest version on Win 10.

[ZZJHONS]

Maybe windows 11 thingy? Location or something?

[paramecie]

Ahhh this. It's Win 11. Here I have instead an up and down triangle. But that doesn't do an addition, it increases and decreases.

Frankly I like this proposal:
#1023 (comment)