Implements FilterTemplate system for event matching

Introduces flexible key-based filtering for calendar events across different view configurations

Adds new FilterTemplate class to:
- Define event matching rules based on view configuration
- Support multi-level grouping (team/resource/date)
- Handle dynamic key generation for columns and events

Enhances view configuration with explicit id properties and derived fields

docs/filter-template.md

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+# FilterTemplate System
+
+## Problem
+
+En kolonne har en unik nøgle baseret på view-konfigurationen (f.eks. team + resource + date).
+Events skal matches mod denne nøgle - men kun på de felter viewet definerer.
+
+## Løsning: FilterTemplate
+
+ViewConfig definerer hvilke felter (idProperties) der indgår i kolonnens nøgle.
+Samme template bruges til at bygge nøgle for både kolonne og event.
+
+**Princip:** Kolonnens nøgle-template bestemmer hvad der matches på.
+
+---
+
+## ViewConfig med idProperty
+
+ViewConfig er kilden til sandhed - den definerer grupper OG deres relations-id property.
+
+```typescript
+interface GroupingConfig {
+  type: string;                 // 'team', 'resource', 'date'
+  values: string[];             // ['EMP001', 'EMP002']
+  idProperty: string;           // property-navn på event (eks. 'resourceId')
+  derivedFrom?: string;         // for date: udledes fra 'start'
+}
+```
+
+### Eksempler
+
+**Team → Resource → Date view:**
+```typescript
+{
+  groupings: [
+    { type: 'team', values: ['team-a'], idProperty: 'teamId' },
+    { type: 'resource', values: ['EMP001', 'EMP002'], idProperty: 'resourceId' },
+    { type: 'date', values: ['2025-12-09', '2025-12-10'], idProperty: 'date', derivedFrom: 'start' }
+  ]
+}
+```
+
+**Simple date-only view:**
+```typescript
+{
+  groupings: [
+    { type: 'date', values: ['2025-12-09', '2025-12-10'], idProperty: 'date', derivedFrom: 'start' }
+  ]
+}
+```
+
+---
+
+## FilterTemplate Klasse
+
+```typescript
+class FilterTemplate {
+  private fields: Array<{
+    idProperty: string;
+    derivedFrom?: string;
+  }> = [];
+
+  addField(idProperty: string, derivedFrom?: string): this {
+    this.fields.push({ idProperty, derivedFrom });
+    return this;
+  }
+
+  buildKeyFromColumn(column: HTMLElement): string {
+    return this.fields
+      .map(f => column.dataset[f.idProperty] || '')
+      .join(':');
+  }
+
+  buildKeyFromEvent(event: ICalendarEvent, dateService: DateService): string {
+    return this.fields
+      .map(f => {
+        if (f.derivedFrom) {
+          return dateService.getDateKey((event as any)[f.derivedFrom]);
+        }
+        return (event as any)[f.idProperty] || '';
+      })
+      .join(':');
+  }
+}
+```
+
+---
+
+## Flow
+
+```
+Orchestrator
+    │
+    ├── Læs ViewConfig.groupings
+    │
+    ├── Byg FilterTemplate fra groupings:
+    │     for (grouping of viewConfig.groupings) {
+    │       template.addField(grouping.idProperty, grouping.derivedFrom);
+    │     }
+    │
+    ├── Kør group-renderers (bygger headers + kolonner)
+    │     └── DateRenderer sætter column.dataset[idProperty] for ALLE grupperinger
+    │
+    └── EventRenderer.render(ctx, template)
+            │
+            └── for each column:
+                    columnKey = template.buildKeyFromColumn(column)
+                    columnEvents = events.filter(e =>
+                        template.buildKeyFromEvent(e) === columnKey
+                    )
+```
+
+---
+
+## Eksempler
+
+### 3-niveau view: Team → Resource → Date
+
+**ViewConfig:**
+```typescript
+groupings: [
+  { type: 'team', values: ['team-a'], idProperty: 'teamId' },
+  { type: 'resource', values: ['EMP001'], idProperty: 'resourceId' },
+  { type: 'date', values: ['2025-12-09'], idProperty: 'date', derivedFrom: 'start' }
+]
+```
+
+**Template:** `['teamId', 'resourceId', 'date']`
+
+**Kolonne-nøgle:** `"team-a:EMP001:2025-12-09"`
+**Event-nøgle:** `"team-a:EMP001:2025-12-09"`
+
+**Match!**
+
+---
+
+### 2-niveau view: Resource → Date
+
+**ViewConfig:**
+```typescript
+groupings: [
+  { type: 'resource', values: ['EMP001'], idProperty: 'resourceId' },
+  { type: 'date', values: ['2025-12-09'], idProperty: 'date', derivedFrom: 'start' }
+]
+```
+
+**Template:** `['resourceId', 'date']`
+
+**Kolonne-nøgle:** `"EMP001:2025-12-09"`
+**Event-nøgle:** `"EMP001:2025-12-09"` (teamId ignoreres - ikke i template)
+
+**Match!**
+
+---
+
+### 1-niveau view: Kun Date
+
+**ViewConfig:**
+```typescript
+groupings: [
+  { type: 'date', values: ['2025-12-09'], idProperty: 'date', derivedFrom: 'start' }
+]
+```
+
+**Template:** `['date']`
+
+**Kolonne-nøgle:** `"2025-12-09"`
+**Event-nøgle:** `"2025-12-09"` (alle andre felter ignoreres)
+
+**Match!** Samme event vises i alle views - kun de relevante felter indgår i matching.
+
+---
+
+## Kerneprincipper
+
+1. **ViewConfig definerer nøgle-template** - hvilke idProperties der indgår
+2. **Samme template til kolonne og event** - sikrer konsistent matching
+3. **Felter udenfor template ignoreres** - event med ekstra felter matcher stadig
+4. **idProperty** - eksplicit mapping mellem gruppering og event-felt
+5. **derivedFrom** - håndterer felter der udledes (f.eks. date fra start)