feat: update empty states across chat views

[oliverlaz]

🎯 Goal

Implement the REACT-811 empty-state updates across channel, message, and thread surfaces so the web SDK matches the new cross-platform empty-state designs.

Linear: REACT-811

🛠 Implementation details

• updated channel empty-state copy to No conversations yet, added the missing No items exist key, and translated the new thread placeholder copy across all supported locales
• switched the channel-list and thread-list empty placeholders to the shared IconBubbles icon and updated the thread-list label to Reply to a message to start a thread
• rendered the message-panel empty state when no channel is selected, while preserving EmptyPlaceholder={null} as an explicit opt-out
• rendered the message-panel empty state in the thread pane when loading has finished and there is no active thread to display
• centered the thread-list placeholder and added the right-side separator so the thread list and thread content panes stay visually distinct
• moved ChannelList styling into the main styling entrypoint and updated the Vite example imports to avoid duplicate ChannelList layout/theme imports
• replaced the SCSS watch command with a Node-based watcher that rebuilds index.css on SCSS changes using the Sass API directly instead of spawning yarn build-styling
• added coverage for the empty-state behavior in Channel, ChatView, and EmptyStateIndicator

🎨 UI Changes

• updated empty-state copy, iconography, centering, and pane separation across channel and thread views
• no screenshots attached

[github-actions]

Size Change: +1.25 kB (+0.3%)

Total Size: 414 kB

Filename	Size	Change
./dist/Channel-CFU2xRc7.js	0 B	-20.6 kB (removed)	🏆
./dist/Channel-MhCKe2pq.js	20.9 kB	+20.9 kB (new file)	🆕
./dist/ChannelPreview-bNlL3AWK.js	8.53 kB	+8.53 kB (new file)	🆕
./dist/ChannelPreview-JwqYRYF-.js	0 B	-8.53 kB (removed)	🏆
./dist/cjs/experimental.js	3.62 kB	+1 B (+0.03%)
./dist/cjs/index.js	210 kB	-136 B (-0.06%)
./dist/css/index.css	37.7 kB	+788 B (+2.14%)
./dist/limits-BfipDeFM.js	0 B	-63.1 kB (removed)	🏆
./dist/limits-DuUuDo7N.js	63.4 kB	+63.4 kB (new file)	🆕

ℹ️ View Unchanged

Filename	Size
./dist/audioProcessing-BbOs2wMd.js	1.32 kB
./dist/cjs/emojis.js	2.97 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

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