feat: UI design system - sidebar layout, workspace system, and shell redesign (#77)

* feat: add workspace system and sidebar layout (UI design system Phase A+B)

Backend: Workspace model, migration (036), schemas, CRUD API endpoints.
Adds workspace_id to trees and categories, seeds 4 default workspaces
per account, auto-assigns existing trees by tree_type.

Frontend: Complete AppLayout rewrite from top-nav to CSS Grid shell
with persistent sidebar + topbar. New components: WorkspaceSwitcher,
NavItem, CategoryList, TagCloud, TopBar, Sidebar. Dashboard components:
QuickStats, FiltersBar, SectionGroup, TreeListItem, SessionsPanel.
WorkspaceStore with localStorage persistence.

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

* feat: add command palette search, dashboard rewrite, and shell height fixes (Phase C)

- Add ⌘K command palette with debounced search across flows and sessions
- Rewrite QuickStartPage as dashboard with stats, filters, sessions panel
- Fix h-[calc(100vh-4rem)] → h-full across all pages for CSS Grid shell
- Add active session count badge to sidebar Sessions nav item

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

* feat: add sidebar collapse, category/tag filtering, and workspace CRUD (Phase D)

- Sidebar collapse/expand toggle with icon-only rail mode (persisted)
- Sidebar category/tag clicks navigate to /trees with URL params
- TreeLibraryPage syncs filters from URL search params bidirectionally
- Workspace create modal with icon picker and auto-slug generation
- TopBar logo adapts to collapsed sidebar state

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

* feat: add Quick Launch modal with actions and recent flows

- Zap button opens Quick Launch with create/navigate shortcuts
- Shows recent flows for quick session start
- Keyboard navigation support (arrows + enter)

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

* feat: add activity notifications panel with session feed

- Bell icon shows dot indicator for recent activity
- Dropdown panel shows recent sessions with status icons
- Links to session detail and sessions list page

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

* feat: remove workspace system, add pinned flows and label renames

Replace workspace system with pinned flows API (pin/unpin/list/reorder).
Rename user-facing labels: Tree→Flow, Procedure→Project. Add sidebar
nav sub-items for flow type filtering. Remove 11 workspace files,
add migrations 037-038, clean all workspace references.

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

* fix: collapsed sidebar layout scaling and toggle button size

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

* refactor: migrate auth pages to new design system

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

* refactor: migrate TreeLibraryPage to new design system

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

* refactor: migrate session pages to new design system

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

* refactor: migrate TreeEditorPage to new design system

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

* refactor: migrate TreeNavigationPage to new design system

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

* refactor: migrate session sharing components to new design system

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

* chore: remove workspace dropdown animation (dead code)

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

* refactor: migrate common components to new design system

Migrate 15 components from monochrome glass-card design to purple gradient
accent design system tokens (bg-card, border-border, text-foreground,
bg-gradient-brand, etc.)

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

* refactor: migrate procedural and step library components to new design system

Migrate 10 components from monochrome glass-card design to purple gradient
accent design system tokens.

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

* refactor: migrate admin pages and components to new design system

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

* refactor: migrate remaining pages to new design system

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

* refactor: migrate remaining components to new design system

Migrates 38 files: tree-editor forms, session modals, step library,
common components, library views, tree preview, and misc UI to use
design tokens (bg-card, border-border, text-foreground, bg-accent,
bg-gradient-brand) replacing old monochrome patterns.

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

* fix: keep brand text visible on sidebar collapse, hide sub-items until hover

- TopBar: always show "ResolutionFlow" text regardless of sidebar state
- NavItem: sub-items (Troubleshooting, Projects) hidden by default,
  revealed on hover or when a child route is active

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/index.css

@@ -27,6 +27,7 @@
     --radius: 0.75rem;
 
     /* App Shell tokens */
+    --sidebar-w: 260px;
     --sidebar-bg: 240 10% 4.5%;
     --sidebar-hover: 240 6% 12%;
     --sidebar-active: 243 75% 59% / 0.08;
@@ -68,6 +69,48 @@
   }
 }
 
+/* App Shell Grid Layout */
+.app-shell {
+  display: grid;
+  grid-template-columns: var(--sidebar-w) 1fr;
+  grid-template-rows: 56px 1fr;
+  height: 100vh;
+  overflow: hidden;
+  transition: grid-template-columns 200ms ease;
+}
+
+.app-shell--collapsed {
+  grid-template-columns: 56px 1fr;
+}
+
+.topbar {
+  grid-column: 1 / -1;
+}
+
+.sidebar {
+  overflow-y: auto;
+}
+
+.main-content {
+  overflow-y: auto;
+}
+
+/* Mobile: single column */
+@media (max-width: 767px) {
+  .app-shell {
+    grid-template-columns: 1fr;
+  }
+}
+
+/* Staggered fade-in for page sections */
+.fade-in {
+  animation: fadeIn 0.3s ease forwards;
+}
+
+@keyframes fadeIn {
+  from { opacity: 0; transform: translateY(6px); }
+  to { opacity: 1; transform: translateY(0); }
+}
 /* Animations */
 @keyframes fade-in {
   from { opacity: 0; }