fix: keep the geohash teleport field focused

Summary

Fixes #1797. The channel sheet re-reads participant counts on a 5-second TimelineView, and that refresh wrapped the geohash text field. Rebuilding the field resigns first responder, so the keyboard drops as soon as the field is tapped and it stays on the gray placeholder.

The field (and quick join, which has no count) now sit outside that ticker. The count rows and bookmarks still refresh. On iOS the list also no longer treats that height change as a scroll that dismisses the keyboard. The sheet uses NavigationStack instead of NavigationView, which was resigning the field inside a sheet.

Test plan

• On iOS, open the location channel sheet, tap the geohash field, and type a hash. The keyboard should stay up for more than a few seconds, including across a count refresh.
• Teleport with a valid geohash still switches channel and closes the sheet.
• Bookmarks and nearby counts still update while the sheet is open.
• Could not run xcodebuild here (no Xcode). CI covers the build.

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