feat: add 90-day no-planter sponsor withdrawal to campaign-funding

[daameeloolaa-png]

• Add created_at (u64) and planter (OptionalAddress) fields to Campaign struct
• Add OptionalAddress enum for Soroban-compatible Option semantics
• Add NINETY_DAYS constant (7_776_000 seconds)
• Add SponsorWithdrew variant to CampaignStatus
• Add new error variants: PlanterAlreadyAssigned(17), WithdrawWindowNotOpen(18), PlanterAssigned(19), NothingToWithdraw(20)
• Add PlanterAssignedEvent and SponsorWithdrewEvent event structs
• Add assign_planter function: creator-only, one-time planter assignment
• Add sponsor_withdraw function: if 90 days pass with no planter assigned, sponsor can withdraw full total_raised with zero protocol fee
• Update create_campaign to set created_at = ledger timestamp, planter = None
• Add 9 tests covering all new paths and edge cases (48 total, all passing)
• Fix pre-existing duplicate soroban-sdk key in contracts/Cargo.toml
• Update should_panic strings to Error(Contract, #N) format (soroban-sdk v27)
  closes feat(contract): Emergency withdrawal - sponsor can withdraw funds if planting deadline missed #660

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[coderabbitai]

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[Utilitycoder]

Thanks for the work. Kindly ensure you offramp with Fundable at https://stellar.fundable.finance/offramp