feat: implement donation enhancements (#195, #196, #197, #198)

[Mkalbani]

#195 - raised_amount update and milestone unlock check:

• After donation, raised_amount updated atomically
• Loop over milestones: if raised_amount >= target_amount && Locked, set Unlocked
• milestone_unlocked event emitted for each newly unlocked milestone
• Multiple milestones can unlock in single donation
• TotalRaised storage updated after each donation
• DonorRecord updated/created with cumulative totals

#196 - get_donor_record view function:

• fn get_donor_record(env, donor) -> Option
• Returns None if address has never donated
• Read-only, no auth required

#197 - get_total_raised view function:

• fn get_total_raised(env) -> i128
• Returns current raised_amount from TotalRaised storage
• No auth required, returns 0 if no donations yet

#198 - goal reached status transition:

• After donation, if raised_amount >= goal_amount, set status = GoalReached
• campaign_goal_reached event emitted
• Does not prevent additional donations (GoalReached still accepts)
• Checked after every donation, only transitions from Active

closes #195
closes #196
closes #197
closes #198

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[ayshadogo]

LGTM

Thank you for your contribution