fix(contract): prevent XP farming, auto-reset streak, expand tests

[Kaylahray]

closes #293
closes #294
closes #298

Solved all three contract issues in one PR.

• Prevent Duplicate Puzzle Submissions and XP Farming #294: Added duplicate-submission guard in submit_puzzle() using a
  (player, puzzle_id) storage key checked before any state mutation.
  Panics with "Puzzle already submitted" on repeat calls.

• Enforce Automatic Streak Reset Based on Inactivity #293: Added last_active_timestamp to Player. On each submission,
  if >24 h has passed since last activity, current_streak auto-resets
  to 0 before incrementing. Guard uses puzzles_solved > 0 to handle
  the Soroban test env starting ledger clock at 0.

• Expand Test Suite to Match Documented Coverage #298: Expanded test suite from 2 → 14 tests covering all documented
  functions, edge cases, and failure scenarios. All 14 pass.

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