fix: handle signed URL download failures

[amanharshx]

Summary

• Detect expired Cloud CDN, GCS V4, and AWS SigV4 image URLs before downloading.
• Improve download-failure details, copy feedback, and selection containment.

Motivation

Expired GCS V4 URLs returned HTTP 400 and were misreported as image-server rejections. Failure details also showed contradictory remedies and awkward copy feedback.

Key Changes

• Preserve Expires support and add X-Goog-* and X-Amz-* expiry parsing with clock-skew tolerance.
• Keep malformed or incomplete expiry metadata on the normal download path.
• Align primary and detailed remedies and clarify truncated filename examples.
• Keep copy status live region mounted and update button labels in place.
• Clip selection painting inside result cards.
• Verified with frontend build, 29 frontend tests, Rust fmt/check/clippy, and 99 Rust tests.
• Sanity check: expired signed URL → expired_url; active or malformed URL → download attempted.
• Risk: AWS coverage is specification-backed and synthetic. WKWebView visual verification remains manual.

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Codecov Report

❌ Patch coverage is 95.13889% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src-tauri/src/downloader.rs	95.68%	5 Missing ⚠️
src/components/converter-screen.tsx	94.73%	1 Missing ⚠️
src/lib/download-failures.ts	88.88%	1 Missing ⚠️

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