Fix OFFLINE_GROWTH: block vine spread and tree growth from saplings

[Copilot]

OFFLINE_GROWTH=off correctly blocked crops but left vines (and other spreading plants) and all tree types growing freely. Two root causes:

1. onSpread(BlockSpreadEvent) had a hard-coded material allowlist (KELP, BAMBOO, BAMBOO_SAPLING) — everything else (vines, cave vines, twisting/weeping vines, glow lichen, …) passed through unchecked.
2. Trees growing from saplings fire StructureGrowEvent, not BlockGrowEvent — no handler existed for it at all.

Changes

OfflineGrowthListener

• Drop the material filter from onSpread entirely; all BlockSpreadEvents on protected islands are now checked. Switch island lookup to e.getSource().getLocation() (the existing plant, always on the island) instead of the destination block which may be outside the protection zone.
• Add onStructureGrow(StructureGrowEvent) handler — covers all tree and mushroom growth from saplings (oak, birch, spruce, jungle, acacia, dark oak, mangrove, …).
• Extract the duplicated per-member online-check into a private checkGrowth(Location, Cancellable) helper shared by all three handlers.

// Before: only three materials were ever blocked
Material m = e.getBlock().getType();
if (!m.equals(Material.KELP) && !m.equals(Material.BAMBOO) && !m.equals(Material.BAMBOO_SAPLING)) {
    return; // vines, weeping vines, etc. all slipped through here
}

// After: all spreading is checked; source location used for island lookup
checkGrowth(e.getSource().getLocation(), e);

OfflineGrowthListenerTest

• Updated testOnSpreadMembersOfflineAnyMaterial (formerly …Tree): now uses Material.VINE and asserts the spread is blocked.
• Added 6 tests for onStructureGrow: flag allowed, member online, member offline, non-island, non-BentoBox world, and bone-meal-triggered growth.