refactor: onboarding, mobile nav, remove maintenance flows

- Onboarding steps guide toward copilot usage, not flow building
- Mobile nav updated to match sidebar (Session History, Guided Flows)
- Remove Step Library from mobile nav
- Remove Maintenance from flow type filter tabs
- Remove Maintenance badge from all tree views (grid, list, table)
- Remove Maintenance create option from CreateFlowDropdown
- Add copilot-first dashboard plan and solutions library spec docs

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

frontend/src/pages/TreeLibraryPage.tsx

@@ -365,7 +365,7 @@ export function TreeLibraryPage() {
             <div className="flex flex-col gap-3 sm:flex-row sm:items-center sm:justify-between">
               {/* Type filter tabs */}
               <div className="flex rounded-lg border border-border p-0.5">
-                {(['all', 'troubleshooting', 'procedural', 'maintenance'] as const).map((t) => (
+                {(['all', 'troubleshooting', 'procedural'] as const).map((t) => (
                     <button
                       key={t}
                       onClick={() => setTypeFilter(t)}