ui(trees): tokenize sidebar header chevrons and finish QueryaIcons

[ZhuchkaTriplesix]

Parent

Part of epic #490. Finishes visual unify from #472 / #475 / #476.

Problem

• Connection header expand chevrons hard-coded size: 16 vs inner QueryaIconSizes.treeExpand (13)
• Folder chevron in sidebar is 18; headers are 16
• Some drivers still use bare Icons.chevron_right_rounded instead of QueryaIcons.expandClosed
• Connection chrome icons use magic 16 (not a size token)

Proposal

• Add QueryaIconSizes.sidebarExpand (and/or sidebarConnectionIcon = 16)
• Use QueryaIcons.expandClosed on all connection/folder headers
• Optional: extract shared _SidebarExpandChevron

Files

• lib/core/ui/querya_icon_sizes.dart, querya_icons.dart
• connections_panel_{postgres_connection,redis,mongo,mysql,sqlite,extension,sidebar}.dart

Acceptance

• Header chevron size shared via token across drivers
• All headers use QueryaIcons.expandClosed
• No hard-coded 16/18 for those chevrons

[ZhuchkaTriplesix]

Shipped on dev (PR targeted dev, auto-close did not fire).