fix: give the eco-challenge error screen a way out (#1105)

[MOHITKOURAV01]

Closes #1105

setError(null) appeared nowhere in this component, so the first failed load ended it for the life of the page: a red box holding whatever string the service threw — for an offline user, the browser's raw TypeError: Failed to fetch — with no button, no link, and no way back other than a reload.

Worse, the paths that do re-fetch could not clear it. handleJoin and handleClaim both await loadData() on success, and loadData never reset the flag on the way in, so those loads populated data behind an error screen nobody could see past.

What changed

• The error is recoverable. Cleared by a successful load; a Try again button on the full-page state; once challenges are on screen, a failure is a banner over them rather than instead of them.
• Empty is not failure. A successful response with no active challenges said nothing at all — just the "Active Challenges" heading above an empty grid. It now says so.
• A partial payload no longer throws. data?.challenges.map(...) stopped its optional chain at data, so a response of {} was undefined.map during render. fetchActiveChallenges returns response.json() unchecked, so nothing ruled that out. readChallengeData normalises the shape.
• The progress bar stays inside its track. Math.min((progress / target) * 100, 100) capped the top and left the divisor unguarded. A targetValue of 0 gives Infinity — NaN when progress is 0 too — and width: "NaN%" is dropped by the browser, so the bar silently vanished rather than looking wrong. Negative and missing values did the same, and progress.progress rendered raw as undefined / 10 reports.
• The stray full stop. A leftover edit rendered ✅ Challenge Completed! Claim your reward. . under every completed challenge — on the one line a user is most likely to read closely.
• window.alert → the page. Join and claim outcomes are reported in-place instead of blocking the tab.

Tests

26 new tests in src/components/EcoChallengeDashboard.test.jsx. progressPercent, progressCount and readChallengeData are asserted directly — including a property-style pass checking that progressPercent returns a finite value in [0, 100] for every hostile input — and the error, empty and malformed-payload paths through the component.

Verification

npx vitest run src/components/EcoChallengeDashboard.test.jsx   # 26 passed
npx eslint src/components/EcoChallengeDashboard.jsx src/components/EcoChallengeDashboard.test.jsx   # clean

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