Revamp /sell GUI, add /sellmulti and /sellworth commands

[Copilot]

Separates multiplier display from the sell GUI into its own menu, simplifies /sell to a clean item-drop inventory with a sell button, and adds a paginated item prices browser.

Changes

/sell — Clean sell inventory

• Now 9×6 with rows 0-4 as free item placement slots
• Bottom-right (slot 53): lime glass pane Sell button that shows total value on hover
• Close-to-sell behavior preserved
• Sell button refreshes dynamically after each item placement via scheduled task

/sellmulti — New multiplier overview

• 1×9 GUI showing all category multipliers (earned, daily boost, effective)
• Clicking a category opens its existing progress snake path
• Category progress/items GUIs now navigate back here instead of /sell

/sellworth — New item prices browser

• Aliases: /worth, /prices, /itemprices
• Paginated 54-slot GUI with all priced items
• Hopper-based filter button cycles through All → each category
• Shows base price, multiplier, effective price, and category per item

Config

• All new labels, titles, materials, and lore fully configurable under messages.shop.*, messages.sellmulti.*, messages.worth-gui.*, and icons.sell-button
• sell-multi-gui and worth-gui top-level config sections added
• Plugin version bumped to 2.2.0