UI polish: sidebar nav, Companies/Issues layout, assignee in issue defaults

- Sidebar: add Approvals and Companies links, remove temporary Design Guide link
- Companies: replace inline create form with 'New Company' button (opens onboarding)
- Issues: move filter tabs inline with page heading (consistent with Agents)
- DialogContext: add assigneeAgentId to NewIssueDefaults
- NewIssueDialog: wire assigneeAgentId default (pre-select assignee when opening
  from agent detail page)
- CommandPalette: minor cleanup
- App: add DesignGuide route (dev-only, no sidebar link)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/context/DialogContext.tsx

@@ -4,6 +4,7 @@ interface NewIssueDefaults {
   status?: string;
   priority?: string;
   projectId?: string;
+  assigneeAgentId?: string;
 }
 
 interface DialogContextValue {