Add US-20 (404 page) to user stories

Catch-all route for unknown paths with link back to home.

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

spec/userstories.md

@@ -4,9 +4,9 @@
 
 ## Status
 
-- Total stories: 20
+- Total stories: 21
 - Complete: 0
-- Remaining: 20
+- Remaining: 21
 
 ## Token Model
 
@@ -467,3 +467,20 @@ The following terms are used consistently across all stories:
 
 **Notes:** The overseer identified that using the expiry date as a deletion mechanism (setting it to today or a past date in US-5) was unintuitive and conflated two different actions. US-5 now enforces that the expiry date can only be set to a future date. If the organizer wants the event gone immediately, they use this explicit deletion feature. Unlike cancellation (US-18), which keeps the event visible with a cancellation notice until the expiry date, deletion removes the event entirely and immediately. This is the organizer's "nuclear option" — useful when the event was created by mistake, contains wrong information, or is no longer needed at all. The deletion behavior is identical to what US-12 does automatically after expiry, but triggered manually and immediately by the organizer.
 
+---
+
+### US-20: 404 page
+
+**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.
+
+**Acceptance Criteria:**
+- [ ] Unknown routes show a "Page not found" message
+- [ ] The page includes a link back to the home page
+- [ ] The page follows the design system
+
+**Dependencies:** None
+
+**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. This is a small UX story but important for polish. Note: This story has no structural dependencies but requires the frontend scaffold from T-4 (which includes T-1) to be practically implementable.
+