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>

backend/app/models/tree.py

@@ -120,7 +120,6 @@ class Tree(Base):
         onupdate=lambda: datetime.now(timezone.utc)
     )
     usage_count: Mapped[int] = mapped_column(Integer, default=0)
-
     # Fork tracking
     parent_tree_id: Mapped[Optional[uuid.UUID]] = mapped_column(
         UUID(as_uuid=True),
@@ -184,7 +183,7 @@ class Tree(Base):
         cascade="all, delete-orphan"
     )
 
-    # New organization relationships
+    # Organization relationships
     category_rel: Mapped[Optional["TreeCategory"]] = relationship("TreeCategory", back_populates="trees")
     tags: Mapped[list["TreeTag"]] = relationship(
         "TreeTag",