Convert grass from tile type to pickable entity

[Copilot]

Grass was implemented as a tile biome rather than an interactive entity, preventing players from gathering and placing it like in the original prototype.

Changes

• Server: Created Grass entity class following Wood/Stone pattern (non-solid, has quantity, uses grass.png texture)
• Server: Added grass spawning to EntityManager with 80% spawn rate to match original implementation coverage
• Server: Updated PlayerController.createEntityFromItemName() to return new Grass() instead of null

Client already had complete Grass entity support with texture mapping. Players can now gather, carry, and place grass as expected.

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[Copilot]

Pull request overview

This PR converts grass from a tile biome type to an interactive pickable entity, restoring functionality from the original prototype where players could gather and place grass.

Changes:

• Created new Grass entity class following the established pattern for collectible resources
• Added grass entity spawning to world generation with 80% coverage rate
• Updated player controller to support placing grass from inventory

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
server/src/main/java/com/preponderous/roam/model/entity/Grass.java	New entity class for grass with quantity tracking, non-solid property, and persistence support
server/src/main/java/com/preponderous/roam/service/EntityManager.java	Added grass spawning logic with 80% spawn rate (320 entities per room) to match original implementation
server/src/main/java/com/preponderous/roam/controller/PlayerController.java	Updated entity creation to return Grass instance instead of null when placing grass from inventory

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