Migrate project artifacts to spec-kit format

- Move cross-cutting docs (personas, design system, implementation phases,
  Ideen.md) to .specify/memory/
- Move cross-cutting research and plans to .specify/memory/research/ and
  .specify/memory/plans/
- Extract 5 setup tasks from spec/setup-tasks.md into individual
  specs/001-005/spec.md files with spec-kit template format
- Extract 20 user stories from spec/userstories.md into individual
  specs/006-026/spec.md files with spec-kit template format
- Relocate feature-specific research and plan docs into specs/[feature]/
- Add spec-kit constitution, templates, scripts, and slash commands
- Slim down CLAUDE.md to Claude-Code-specific config, delegate principles
  to .specify/memory/constitution.md
- Update ralph.sh with stream-json output and per-iteration logging
- Delete old spec/ and docs/agents/ directories
- Gitignore Ralph iteration JSONL logs

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

specs/026-404-page/spec.md

@@ -0,0 +1,48 @@
+# Feature Specification: 404 Page
+
+**Feature**: `026-404-page`
+**Created**: 2026-03-06
+**Status**: Draft
+**Source**: Migrated from spec/userstories.md
+
+## User Scenarios & Testing
+
+### User Story 1 - See a helpful error page for unknown URLs (Priority: P1)
+
+As a user who navigates to a non-existent URL, I want to see a helpful error page so that I can find my way back instead of seeing a blank screen.
+
+**Why this priority**: Without this, users see a blank page on invalid routes. Basic UX hygiene.
+
+**Independent Test**: Navigate to any non-existent path (e.g. `/does-not-exist`) and verify the error page renders with a way back.
+
+**Acceptance Scenarios**:
+
+1. **Given** a user navigates to a URL that does not match any defined route, **When** the page loads, **Then** a "Page not found" message is displayed.
+2. **Given** the 404 page is displayed, **When** the user looks for navigation, **Then** a link back to the home page is visible and functional.
+3. **Given** the 404 page is displayed, **When** its appearance is evaluated, **Then** it follows the project design system (Electric Dusk + Sora).
+
+### Edge Cases
+
+- What happens when the URL contains special characters or very long paths? The catch-all route should still match and display the 404 page.
+- What happens when JavaScript is disabled? [NEEDS EXPANSION — SPA limitation]
+
+## Requirements
+
+### Functional Requirements
+
+- **FR-001**: The Vue Router MUST include a catch-all route that matches any undefined path.
+- **FR-002**: The catch-all route MUST render a dedicated 404 component with a "Page not found" message.
+- **FR-003**: The 404 page MUST include a link back to the home page.
+- **FR-004**: The 404 page MUST follow the design system defined in `.specify/memory/design-system.md`.
+
+## Success Criteria
+
+### Measurable Outcomes
+
+- **SC-001**: Navigating to any undefined route displays the 404 page instead of a blank screen.
+- **SC-002**: The home page link on the 404 page navigates back to `/` successfully.
+- **SC-003**: The 404 page passes WCAG AA contrast requirements.
+
+**Dependencies:** None (requires frontend scaffold from T-1/T-4 to be practically implementable)
+
+**Notes:** Identified during US-1 post-review. Navigating to an unknown path currently shows a blank page because the Vue Router has no catch-all route.