[Flapjacck]

Summary

Implements the "Apply Hand Scale" feature requested in #3, fixes two GUI bugs
introduced during development, and corrects a crash caused by a wrong mixin
target method name.

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Changes

New feature — Apply Hand Scale toggle

A new "Apply Hand Scale: ON/OFF" toggle button has been added to the settings
panel, styled identically to the existing "Ignore Haste" button.

• When OFF (default): item scale only affects the held item model. The
  player's bare arm renders at vanilla size. This is the existing behaviour.
• When ON: the bare-hand arm mesh also scales to match HELD_ITEM_SCALE.
  This is visible when holding nothing. When holding an item, the arm geometry
  is rendered by a separate entity-renderer pipeline that is outside the scope
  of ItemInHandRenderer; the item itself still scales correctly in all cases.

Config persistence

ItemTweaksConfig gains a new APPLY_HAND_SCALE boolean field (default
false). It is read from and written to config/swing.json alongside the
existing fields, and is reset by the Reset button.

New mixin — ItemInHandRendererArmMixin

Targets AvatarRenderer#renderRightHand and AvatarRenderer#renderLeftHand
(the only methods that render the first-person arm mesh). Applies a
push/scale/pop around each call when APPLY_HAND_SCALE is enabled.

GUI scrolling — rewritten, no longer broken

The previous scroll implementation called this.init() inside
mouseScrolled, which re-added every widget to the list on each tick without
clearing it first. This caused widgets to stack on top of each other
(visible as duplicated/overlapping controls).

The scroll system has been replaced with an in-place repositioning approach:

• Widgets are created once in init().
• A repositionScrollableWidgets() method calls setX()/setY() directly
  on each widget reference and sets widget.visible = false for any widget
  fully outside the clipping window.
• mouseScrolled calls repositionScrollableWidgets() — never init().
• scrollOffset is clamped to [0, MAX_SCROLL] using Mth.clamp.

Known limitation

"Apply Hand Scale" does not visually affect the arm while the player is holding
an item. When holding an item, the arm skin is rendered by the entity-renderer
pipeline (LivingEntityRenderer → player model layers), which is entirely
separate from ItemInHandRenderer. Scaling the arm in that context would
require injecting into the entity renderer and is out of scope for this PR.

Closes #3