fix: wrap attachment previews in message composer

[oliverlaz]

🎯 Goal

Make attachment previews in the message composer wrap onto new lines instead of forcing a horizontal scroll area, so the composer grows with its content for mouse users.

Linear ticket: https://linear.app/stream/issue/REACT-841/make-attachment-preview-container-grow-horizontally

🛠 Implementation details

• remove horizontal scrolling from the attachment preview list and enable wrapping
• adjust attachment preview sizing so file and audio tiles can shrink within the composer width instead of reintroducing overflow
• add a stable attachment-preview-list test hook and update the related tests and snapshot
• validate with yarn build-styling, targeted AttachmentPreviewList tests, and prettier --check
• note that broader MessageInput.test.js coverage is currently failing in this branch before attachment assertions because useMediaRecorder resolves useMessageComposer as non-functional

🎨 UI Changes

No screenshots attached from the CLI environment. This is a composer layout change: attachment previews now wrap to new rows and the input grows vertically instead of showing a horizontal preview scrollbar.

[github-actions]

Size Change: +81 B (+0.02%)

Total Size: 427 kB

Filename	Size	Change
./dist/cjs/index.js	214 kB	+30 B (+0.01%)
./dist/css/index.css	40.3 kB	+51 B (+0.13%)

ℹ️ View Unchanged

Filename	Size
./dist/audioProcessing-BbOs2wMd.js	1.32 kB
./dist/Channel-iBbX1_YS.js	20.2 kB
./dist/ChannelPreview-CXRaPTt0.js	13.6 kB
./dist/cjs/emojis.js	2.97 kB
./dist/cjs/experimental.js	3.7 kB
./dist/cjs/mp3-encoder.js	1.27 kB
./dist/css/v2/emoji-mart.css	1.84 kB
./dist/css/v2/emoji-replacement.css	300 B
./dist/css/v2/index.css	39.4 kB
./dist/css/v2/index.layout.css	22.8 kB
./dist/limits-CxyAOSJp.js	65.2 kB

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