implement quest query and filtering system with pagination

[yusuftomilola]

Summary

• Adds get_quests_by_status(), get_quests_by_creator(), get_active_quests(), and get_quests_by_reward_range() query functions with
  offset/limit pagination support
• Maintains a QuestIds index in storage so quests can be iterated without scanning all keys
• Fixes corrupted/incomplete code in quest.rs, lib.rs, types.rs, events.rs, and init.rs left from prior merge conflicts
• Adds missing storage helpers (is_initialized, mark_initialized, get_admin, set_contract_admin, get_version, set_version, get_config, set_config, get_platform_stats, get_creator_stats)
• Exposes get_platform_stats, get_creator_stats, and reset_platform_stats contract methods

closes #106