feat(ui): redesign sidebar welcome screen and add missing translations

[SameDesu123]

PR Checklist

• Required Checks
  • Have you added type definitions?
  • Have you tested your changes?
  • Have you checked that it won't break any existing features?
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  • Have you checked if it works normally in all models?
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Summary

This PR modernizes the empty state "Welcome" screen in the sidebar and ensures all localization files are fully synchronized with the newly added text keys.

Related Issues

None

Changes

• UI Redesign: Replaced the static grayscale logo in Sidebar.svelte with a glowing Sparkles Lucide icon.
• Aura Effect: Added a beautifully smooth, blurred cyan-to-blue gradient backdrop (blur-3xl) behind the icon to match the Risu theme without sharp boundaries.
• Localization Sync: Added the missing welcomeToRisuai and selectBotToStartChatting translation keys to cn.ts, zh-Hant.ts, de.ts, es.ts, and vi.ts.

Impact

• No impact on existing functionality or chat behavior.

Additional Notes

The glowing effect uses heavily blurred Tailwind CSS utilities and seamlessly assimilates into the dark background, ensuring a premium aesthetic without clashing with dynamic themes.

Preview

In dark mode	In white mode

[SameDesu123]

Also, change the hardcoded RisuAI to Risuai.
The aura effect colors are extracted from the Risuai logo.

[cubicj]

Code-wise this looks fine. The new language keys are synced across locale files, and check, tests, and build pass locally.

This is mostly a design change, so I think the final call should follow the maintainer's preference. My only subjective note is that the welcome text wraps while the sidebar width shrinks during the closing animation, so the transition feels a little less clean.

Approving — thanks for the contribution!