Add temporal grouping to event list (Today/This Week/Next Week/Later/Past)

Group events into five temporal sections with section headers, date subheaders,
and context-aware time display (clock time for upcoming, relative for past).
Includes new useEventGrouping composable, SectionHeader and DateSubheader
components, full unit and E2E test coverage.

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

specs/010-event-list-grouping/data-model.md

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+# Data Model: Event List Temporal Grouping
+
+**Feature**: 010-event-list-grouping | **Date**: 2026-03-08
+
+## Existing Entities (no changes)
+
+### StoredEvent
+
+**Location**: `frontend/src/composables/useEventStorage.ts`
+
+| Field | Type | Description |
+|-------|------|-------------|
+| `eventToken` | `string` | UUID v4, unique identifier |
+| `organizerToken` | `string?` | UUID v4, present if user is organizer |
+| `title` | `string` | Event title |
+| `dateTime` | `string` | ISO 8601 with UTC offset (e.g., `"2026-03-15T20:00:00+01:00"`) |
+| `expiryDate` | `string` | ISO 8601 expiry date |
+| `rsvpToken` | `string?` | Present if user has RSVP'd |
+| `rsvpName` | `string?` | Name used for RSVP |
+
+**Note**: No changes to `StoredEvent`. The `dateTime` field is the sole input for all grouping and sorting logic.
+
+## New Types (frontend only)
+
+### SectionKey
+
+```typescript
+type SectionKey = 'today' | 'thisWeek' | 'later' | 'past'
+```
+
+Enum-like union type for the four temporal sections. Ordering is fixed: today → thisWeek → later → past.
+
+### EventSection
+
+```typescript
+interface EventSection {
+  key: SectionKey
+  label: string          // Display label: "Today", "This Week", "Later", "Past"
+  dateGroups: DateGroup[]
+  emphasized: boolean    // true only for 'today' section
+}
+```
+
+Represents one temporal section in the grouped list. Sections with no events are omitted entirely (never constructed).
+
+### DateGroup
+
+```typescript
+interface DateGroup {
+  dateKey: string        // YYYY-MM-DD (for keying/dedup)
+  label: string          // Formatted via Intl.DateTimeFormat, e.g., "Wed, 12 Mar"
+  events: StoredEvent[]  // Events on this date, sorted by time
+  showSubheader: boolean // false for "Today" section (FR-005)
+}
+```
+
+Groups events within a section by their specific calendar date. The `showSubheader` flag controls whether the date subheader is rendered (always false in "Today" section per FR-005).
+
+## Grouping Algorithm
+
+```
+Input: StoredEvent[], now: Date
+Output: EventSection[]
+
+1. Compute boundaries:
+   - startOfToday = today at 00:00:00 local
+   - endOfToday = today at 23:59:59.999 local
+   - endOfWeek = next Sunday at 23:59:59.999 local (or today if today is Sunday)
+
+2. Classify each event by dateTime:
+   - dateTime < startOfToday → "past"
+   - startOfToday ≤ dateTime ≤ endOfToday → "today"
+   - endOfToday < dateTime ≤ endOfWeek → "thisWeek"
+   - dateTime > endOfWeek → "later"
+
+3. Within each section, group by calendar date (YYYY-MM-DD)
+
+4. Sort:
+   - today/thisWeek/later: date groups ascending, events within group ascending by time
+   - past: date groups descending, events within group descending by time
+
+5. Emit only non-empty sections in fixed order: today, thisWeek, later, past
+```
+
+## State Transitions
+
+None. Events are static data in localStorage. Temporal classification is computed on each render based on current time. No event mutation occurs.
+
+## Validation Rules
+
+No new validation. Existing `isValidStoredEvent()` in `useEventStorage.ts` already validates the `dateTime` field as a parseable ISO 8601 string.