fix(ui): CDN query re-coding

[renefloor]

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Description of the pull request

Every image attachment fails to load (403). Text, PDFs, avatars, reactions, read receipts all fine — images only, and nothing gets logged because the image cache swallows the error.

Root cause: stream_image_cdn.dart built the resized URL with Uri.replace(queryParameters: {...}), which re-encodes existing query values. The signing params are base64, so = padding became %3D and the signature no longer matched.

Why prod never showed it: the two CDNs sign differently — production CloudFront uses URL-safe base64 (__, ~, -, no = at all), so re-encoding was a no-op. GCP Cloud CDN uses standard base64 with = padding. The bug was always there; the CDN migration just exposed it.

Verified by curl: original URL 200, with resize params and = intact 200, with =→%3D 403. Fixed by rebuilding from the raw query string; all images including GIFs now render on device.

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