Fix #438: Zero effective_amount_raised in cancel_campaign - #718

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this is the more complete of the two PRs for #438 (the other being #604). zeroing effective_amount_raised in both cancel_campaign and admin_cancel_campaign is the fix, and crucially you also guarded claim_refund (src/contributions.rs:281) with if !campaign.is_cancelled so post-cancel refunds don't drive the field negative. #604 misses that guard, so i'm inclined to take this one.

a few asks:

1. PR body: it says "closes [Security] claim_creator_revenue: revenue pool depleted by precision loss silently under-pays last claimant #451" but nothing here touches the revenue claim path, that issue is being handled in Fix #451: creator absorbs remaining pool as final revenue claimant #723. please drop that line so [Security] claim_creator_revenue: revenue pool depleted by precision loss silently under-pays last claimant #451 doesn't auto-close on merge.
2. src/campaigns/cancel.rs / src/contributions.rs: no tests. a regression test that cancels a funded campaign, asserts effective_amount_raised == 0, then claims a refund and asserts it stays 0 (and the refund succeeds) would lock in exactly the behavior this PR exists for.
3. CI: the red checks are stale, fmt/clippy died on file for module found at both src/campaigns.rs and src/campaigns/mod.rs, which was the broken main from Feature/campaign tag filtering #618 that has since been fixed upstream. a rebase should make all three jobs green without any change on your side.

gate: git fetch upstream && git rebase upstream/main && git push --force-with-lease, add the regression test, and fix the PR body. once CI is green this is good to merge, and #604 can be closed as superseded.