fix(RankedBallot): Validate candidate ID bounds and uniqueness in vote()

[Muneerali199]

Summary

Fixes two silent security vulnerabilities in RankedBallot.vote() that could corrupt election results.

Closes #244
Closes #228

---

Problem

Bug 1 — No bounds check on candidate IDs

If a voter submitted a voteArr containing a value >= totalCandidates, the contract would attempt to write to candidateVotes[voteArr[i]] out-of-bounds, causing an unguarded Solidity panic (array-out-of-bounds revert) with no descriptive error. Malformed input was not rejected cleanly.

Bug 2 — No uniqueness check (vote manipulation)

A voter could submit a ranking like [0, 0, 0] — the same candidate ID repeated across all positions. Since no uniqueness validation existed, the candidate at index 0 would receive points for every rank (n + (n-1) + (n-2) + ...), while all other candidates received zero. This silently corrupted results without any revert.

---

Fix

Added a bool[] memory seen tracking array in vote():

// Before
for (uint i = 0; i < totalCandidates; i++) {
    candidateVotes[voteArr[i]] += totalCandidates - i;
}

// After
bool[] memory seen = new bool[](totalCandidates);
for (uint i = 0; i < totalCandidates; i++) {
    uint candidateId = voteArr[i];
    if (candidateId >= totalCandidates) revert InvalidCandidateID();
    if (seen[candidateId]) revert DuplicateCandidateID();
    seen[candidateId] = true;
    candidateVotes[candidateId] += totalCandidates - i;
}

Two new typed errors added to Errors.sol:

error InvalidCandidateID();
error DuplicateCandidateID();

Both errors also registered in client/app/helpers/ErrorMessage.ts per contributing guidelines.

---

Files Changed

File	Change
blockchain/contracts/ballots/RankedBallot.sol	Added bounds check + uniqueness check in vote()
blockchain/contracts/ballots/interface/Errors.sol	Added InvalidCandidateID, DuplicateCandidateID
client/app/helpers/ErrorMessage.ts	Registered both new error messages

---

Checklist

• Fix targets a specific, reproducible bug (not style/lint)
• New custom errors follow existing Errors.sol interface pattern
• ErrorMessage.ts updated per contributing guidelines
• No unrelated changes included
• Branch created fresh from upstream/main

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Reviewing files that changed from the base of the PR and between e4df55f and 884a791.

📒 Files selected for processing (3)

• blockchain/contracts/ballots/RankedBallot.sol
• blockchain/contracts/ballots/interface/Errors.sol
• client/app/helpers/ErrorMessage.ts

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