feat(management): extract repository management into dashboard component

Move the inline repository management UI into a new RepositoryDashboard component
and wire it into the application. Refactor app state to compute a reactive
dashboardRepos list and update tab-closing logic so tabs can be closed while
preserving the management view when appropriate.

- Add a full dashboard with search, categories, workspaces and persisted prefs.
- Refactor close behavior to support keeping the management view open.
- Improve UI/CSS and accessibility for the management tab (compact layout and label).

src/lib/components/SelectMenu.svelte

@@ -50,7 +50,8 @@
     const rect = trigger.getBoundingClientRect();
     const viewportGap = 8;
     const menuGap = 5;
-    const desiredHeight = Math.min(300, options.length * 34 + 24);
+    const groupHeaderCount = new Set(options.map((option) => option.group).filter(Boolean)).size;
+    const desiredHeight = Math.min(300, options.length * 32 + groupHeaderCount * 36 + 12);
     const spaceBelow = window.innerHeight - rect.bottom - viewportGap;
     const spaceAbove = rect.top - viewportGap;
     const openAbove = spaceBelow < Math.min(desiredHeight, 180) && spaceAbove > spaceBelow;