Fix Eikthyr shard rarity scaling

[jneb802]

Problem

Eikthyr Shocking Charge stores a different value for each shard rarity, but the runtime damage calculation does not use that value. Every rarity therefore releases 30% of banked damage after 15 hits.

Changes

• Scale the released damage from the configured shard value.
• Preserve Epic at the existing 30% baseline.
• Use 20%, 30%, 40%, 50%, and 60% for Rare through Ancient with the default values.
• Show the damage percentage and required hit count in the tooltip.
• Add a configurable reference value for custom server rarity ramps.

Validation

• Reproduced the fault by tracing the configured rarity values through the equipped-effect lookup into the discharge calculation.
• Built EpicLoot on macOS against the local Valheim assemblies.
• Build result: 0 errors.

The build reports existing environment warnings for the Windows-style Managed folder check and missing mod-tools.