feat: configurable arrow key step size for selection widget

[AnveshJarabani]

Summary

• Adds two new config keys (arrowMoveStep and arrowResizeStep) that control how many pixels each arrow key press moves/resizes the selection
• Both exposed in the GUI settings (General tab) as spinboxes with range 1-200
• Default value of 1 preserves existing behavior — fully backwards compatible

Motivation

The selection widget currently uses hardcoded 1px steps for arrow key movement and resizing. On modern high-resolution displays, this makes keyboard-driven region selection impractical — you'd need hundreds of key presses to move a selection across the screen.

This addresses the keyboard-only workflow requested in #3570. While it doesn't add full keyboard-only region creation, it makes the arrow key adjustment significantly more usable by letting users configure their preferred step size.

Changes

File	Change
src/utils/confighandler.cpp	Register arrowMoveStep and arrowResizeStep as BoundedInt(1, 200, 1)
src/utils/confighandler.h	Add getter/setter macros
src/widgets/capture/selectionwidget.h	Add m_moveStep and m_resizeStep members
src/widgets/capture/selectionwidget.cpp	Load step sizes from config, use in all 12 move/resize/symResize methods
src/config/generalconf.h	Declare spinbox widgets and slots
src/config/generalconf.cpp	Add spinbox UI in General tab with tooltips
flameshot.example.ini	Document new keys

Usage

In flameshot.ini:

[General]
arrowMoveStep=10
arrowResizeStep=5

Or via the GUI: Settings → General → "Arrow move step (px)" / "Arrow resize step (px)"

Test plan

• Default values (1) preserve existing 1px behavior
• Setting arrowMoveStep=10 makes arrow keys move selection by 10px
• Setting arrowResizeStep=20 makes Shift+arrow resize by 20px
• GUI spinboxes read and write values correctly
• Values persist across restarts
• Boundary clamping works (selection doesn't go off-screen)
• Minimum selection size (1x1) is still enforced

Closes #3570