feat: add WannPassts booking app with Go backend and Vue frontend

Initial project setup for a privacy-focused, self-hosted scheduling app:

- Go backend with JWT auth, SQLite storage, and chi router
- Calendar integrations via Google OAuth (FreeBusy), CalDAV, and ICS links
- Public booking pages with configurable slots and booking requests
- AES-256-GCM encryption for stored provider tokens
- Vue 3 + TypeScript frontend with Vite, Pinia, and Vue Router
- Docs, .gitignore, and .env.example for local development

backend/internal/calendar/provider.go

@@ -0,0 +1,41 @@
+package calendar
+
+import (
+	"context"
+	"sort"
+	"time"
+)
+
+// Interval ist ein "beschäftigt"-Zeitraum. Enthält bewusst KEINE Termindetails,
+// nur Start und Ende – mehr verlässt den Kalender des Nutzers nie.
+type Interval struct {
+	Start time.Time
+	End   time.Time
+}
+
+type Provider interface {
+	// FetchBusy liefert alle busy-Zeiträume im Fenster [from, to).
+	FetchBusy(ctx context.Context, from, to time.Time) ([]Interval, error)
+}
+
+// clampIntervals schneidet Intervalle aufs Fenster zurecht, verwirft ungültige
+// und sortiert das Ergebnis.
+func clampIntervals(in []Interval, from, to time.Time) []Interval {
+	out := make([]Interval, 0, len(in))
+	for _, iv := range in {
+		if !iv.End.After(iv.Start) {
+			continue
+		}
+		if iv.Start.Before(from) {
+			iv.Start = from
+		}
+		if iv.End.After(to) {
+			iv.End = to
+		}
+		if iv.End.After(iv.Start) {
+			out = append(out, iv)
+		}
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].Start.Before(out[j].Start) })
+	return out
+}