feat: implement plugin architecture and application settings with Svelte UI

- Added plugin base and loader for backend extensibility
- Implemented application settings management with config persistence
- Created Svelte-based frontend with Dashboard and Settings pages
- Added API routes for plugins, tasks, and settings
- Updated documentation and specifications
- Improved project structure and developer tools

.specify/templates/spec-template.md

@@ -95,12 +95,6 @@
 - **FR-006**: System MUST authenticate users via [NEEDS CLARIFICATION: auth method not specified - email/password, SSO, OAuth?]
 - **FR-007**: System MUST retain user data for [NEEDS CLARIFICATION: retention period not specified]
 
-### System Invariants (Constitution Check)
-*Define immutable constraints that will become `@INVARIANT` or `@CONSTRAINT` tags in Module Headers.*
-
-- **INV-001**: [e.g., "No direct database access from UI layer"]
-- **INV-002**: [e.g., "All financial calculations must use Decimal type"]
-
 ### Key Entities *(include if feature involves data)*
 
 - **[Entity 1]**: [What it represents, key attributes without implementation]