Balance updates, UI polish, crafting mechanics changes

Balance changes:

• Skeletal mage boss now has more health but deals significantly less damage with the sword.
• Mecha golem final boss now has more health, but the attack speed is significantly reduced.

UI changes:

• Changed the crafting UI right page: the stats panel is now graphically consistent with the rest of the page.

Crafting mechanics:

• Removed the possibility of improving range, attack speed, and damage separately.
• Added the concept of weapon tier, which boosts the aforementioned stats all at once.