feat: reuse channel list loading skeletons for thread list

[oliverlaz]

🎯 Goal

REACT-826: align thread-related loading states with the redesigned skeleton system.

🛠 Implementation details

• render LoadingChannels for the initial ThreadList load when the thread manager is loading and there are no threads yet
• keep the existing ThreadListLoadingIndicator footer behavior for pagination / load-more states
• constrain LoadingChannel message placeholders to the default message-list content width so the skeleton matches the loaded layout more closely
• add focused ThreadList tests covering both the initial loading branch and the loaded list branch

🎨 UI Changes

• Reuses the existing ChannelList loading skeleton styling for ThreadList
• Tightens the LoadingChannel message skeletons to the standard message-list column width
• No new screenshots attached

[github-actions]

Size Change: +73 B (+0.02%)

Total Size: 426 kB

Filename	Size	Change
./dist/Channel-B4DpnmXB.js	0 B	-20.2 kB (removed)	🏆
./dist/Channel-iBbX1_YS.js	20.2 kB	+20.2 kB (new file)	🆕
./dist/cjs/index.js	213 kB	+76 B (+0.04%)
./dist/css/index.css	40.2 kB	-4 B (-0.01%)

ℹ️ View Unchanged

Filename	Size
./dist/audioProcessing-BbOs2wMd.js	1.32 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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