Fix channel header dropdown positioning in the sticky layout

[SnowCait]

Summary

Fix the channel header overflow menu positioning while the header is sticky.

Changes

• Configure the Melt UI dropdown to use Floating UI's fixed positioning strategy.
• Align the menu to the bottom end of the trigger.
• Keep the existing scroll-prevention behavior unchanged.

Root cause

The channel header uses position: sticky, while the dropdown previously relied on Floating UI's default absolute positioning strategy. This combination can cause the floating menu to become mispositioned relative to its trigger.

Impact

The channel menu should remain anchored to the three-dot trigger on both desktop and mobile layouts, including after the header becomes sticky.

Validation

• Confirmed the branch changes only ChannelHeader.svelte.
• Confirmed the dropdown configuration includes placement: 'bottom-end' and strategy: 'fixed'.
• Browser interaction testing is still required in the deployment preview.

Closes #2259

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	nostter	fd73369	Commit Preview URL Branch Preview URL	Jul 15 2026, 12:27 AM