icons from extensions, settings-based tab ordering, static context tabs

Icons now declared on TabContribution via the icon field — the
hardcoded switch in _BottomRail is a fallback only. All sidebar
and context extensions set their Phosphor icon explicitly.

PanelRegistry supports setTabOrder(slot, List<String>) — tabs
render in the order specified by project.layout.sidebar.order /
context.order in settings.yaml. Falls back to registration order.
Priority field removed from all contributions.

Context tabs are static again — persist with their last content,
selection just loads new data and switches focus. No spawn/despawn.

Tickets sidebar priority set to -200 (leftmost, default open).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/builtin/files/src/extension.dart

@@ -1,6 +1,7 @@
 import 'package:clide/builtin/files/src/file_tree_view.dart';
 import 'package:clide/extension/extension.dart';
 import 'package:clide/kernel/kernel.dart';
+import 'package:clide/widgets/widgets.dart';
 
 /// Workspace filesystem panel. Contributes a sidebar tab that renders
 /// the workspace file tree rooted at the git root, powered by the
@@ -22,6 +23,7 @@ class FilesExtension extends ClideExtension {
           id: 'files.tree',
           slot: Slots.sidebar,
           title: 'Files',
+          icon: PhosphorIcons.folder,
           titleKey: 'tab.title',
           i18nNamespace: id,
           priority: -100,