feat: integrate SvelteKit for seamless navigation and improved data loading

specs/004-integrate-svelte-kit/data-model.md

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+# Data Model: SvelteKit Integration
+
+## Entities
+
+### Route
+Represents a navigable URL in the application.
+
+| Field | Type | Description |
+|-------|------|-------------|
+| `path` | String | The URL path (e.g., `/`, `/settings`) |
+| `component` | Svelte Component | The page component to render |
+| `data_requirements` | List<API Endpoint> | Backend data needed for this route |
+| `layout` | Layout | The layout wrapping this route |
+
+**Validation Rules**:
+- `path` must be unique.
+- `path` must follow SvelteKit file-based routing conventions.
+
+### Layout
+Represents a shared UI structure.
+
+| Field | Type | Description |
+|-------|------|-------------|
+| `name` | String | Identifier for the layout (e.g., `default`) |
+| `components` | List<Svelte Component> | Shared components (Header, Footer, Sidebar) |
+| `slot` | Placeholder | Where the route content is injected |
+
+## State Transitions
+
+### Navigation
+1.  **Trigger**: User clicks link or `goto(path)` is called.
+2.  **Action**: SvelteKit router intercepts the request.
+3.  **Data Fetching**: `load` function in `+page.ts` or `+layout.ts` is executed.
+4.  **Rendering**: The new page component is rendered within the layout.
+5.  **URL Update**: Browser history is updated.
+
+### Error Handling
+1.  **Trigger**: Navigation to non-existent path or API failure.
+2.  **Action**: SvelteKit renders `+error.svelte`.
+3.  **Display**: User-friendly error message with recovery options.