feat: Add custom step creation and backend support (Phase 3A: B.8-B.10, B.13)

Implements custom step creation forms and backend persistence:

Task B.8 - StepForm Component:
- Comprehensive form for creating custom steps
- Step type selection (decision/action/solution) with descriptions
- Required fields: title, instructions (markdown supported)
- Optional fields: help text, commands (dynamic array), category, tags
- Visibility control (private/team/public)
- Save to library checkbox
- Full validation with error display
- Dynamic command management (add/remove, label + command)
- Tag input with Enter key support

Task B.9 - CustomStepModal:
- Tabbed modal interface
- Tab 1: "Type My Own" - embeds StepForm
- Tab 2: "Browse Library" - embeds StepLibraryBrowser
- Handles both saved steps (API) and drafts (no save)
- Loading states during step creation
- Error handling with user feedback
- Returns Step or CustomStepDraft to parent

Task B.10 - Backend Custom Steps Support:
- Database migration: add custom_steps JSONB column to sessions
- Updated Session model with custom_steps field
- Updated SessionResponse schema with custom_steps
- Updated SessionUpdate schema to accept custom_steps
- Migration ready to run: 4cdb5cba1aff

Task B.13 - Session Types Updates:
- Added CustomStep and CustomStepDraft interfaces
- Updated Session interface with custom_steps field
- Updated SessionUpdate interface
- Exported step types from types/index.ts
- Full TypeScript support for custom step integration

Remaining tasks: B.11 (TreeNavigationPage integration), B.12 (Export)
Build tested successfully.

Related: Issues #8, #9, #10

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/app/models/session.py

@@ -30,6 +30,7 @@ class Session(Base):
     tree_snapshot: Mapped[dict[str, Any]] = mapped_column(JSONB, nullable=False)
     path_taken: Mapped[list[str]] = mapped_column(JSONB, nullable=False, default=list)
     decisions: Mapped[list[dict[str, Any]]] = mapped_column(JSONB, nullable=False, default=list)
+    custom_steps: Mapped[list[dict[str, Any]]] = mapped_column(JSONB, nullable=False, default=list)
     started_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),
         default=lambda: datetime.now(timezone.utc),