feat: redesign ChannelList loading skeletons

[oliverlaz]

🎯 Goal

Redesign the ChannelList loading skeletons to match the updated mock and the current ChannelPreview footprint.

Ticket: REACT-826

🛠 Implementation details

• add dedicated LoadingChannels.scss and wire it into the loading styling bundle
• update LoadingChannels markup to use the ChannelList preview skeleton structure and render 5 placeholders
• match the skeleton sizing to the current ChannelPreview layout (48px avatar, spacing-md gap, spacing-sm padding)
• add horizontal shimmer styling, row separators, and a shorter top bar over a longer bottom bar
• update the loading snapshot to reflect the new skeleton structure

🎨 UI Changes

Screen.Recording.2026-03-09.at.13.21.26.mov

[github-actions]

Size Change: +354 B (+0.08%)

Total Size: 424 kB

Filename	Size	Change
./dist/cjs/index.js	217 kB	+67 B (+0.03%)
./dist/css/index.css	40 kB	+287 B (+0.72%)

ℹ️ View Unchanged

Filename	Size
./dist/audioProcessing-BbOs2wMd.js	1.32 kB
./dist/Channel-hPAmkyH6.js	20.9 kB
./dist/ChannelPreview-BfFUdUlU.js	8.82 kB
./dist/cjs/emojis.js	2.97 kB
./dist/cjs/experimental.js	3.62 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-LQeJMx9B.js	64.1 kB

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