feat(PROJ-17): Admin Dashboard Systemauslastung immer anzeigen

- Systemauslastungs-Sektion wird immer gerendert (nicht nur bei Erfolg)
- Fehlermeldung wenn /api/admin/system/stats nicht erreichbar ist
- Feature-Status auf In Review gesetzt

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

internal/imap/client.go

@@ -0,0 +1,99 @@
+package imap
+
+import (
+	"crypto/tls"
+	"fmt"
+	"strings"
+
+	imapv2 "github.com/emersion/go-imap/v2"
+	"github.com/emersion/go-imap/v2/imapclient"
+)
+
+// FolderInfo describes a single IMAP folder with exclusion metadata.
+type FolderInfo struct {
+	Name     string `json:"name"`
+	Excluded bool   `json:"excluded"`
+	Reason   string `json:"reason,omitempty"`
+}
+
+// junkTrashNames lists well-known junk/trash folder names for fallback detection.
+var junkTrashNames = []string{
+	"junk", "spam", "trash", "deleted items",
+	"deleted messages", "papierkorb", "gelöschte elemente",
+}
+
+// Connect establishes an IMAP client connection using the specified TLS mode.
+func Connect(host string, port int, tlsMode string) (*imapclient.Client, error) {
+	addr := fmt.Sprintf("%s:%d", host, port)
+
+	switch tlsMode {
+	case "ssl":
+		c, err := imapclient.DialTLS(addr, &imapclient.Options{
+			TLSConfig: &tls.Config{ServerName: host},
+		})
+		if err != nil {
+			return nil, fmt.Errorf("imap connect ssl: %w", err)
+		}
+		return c, nil
+	case "starttls":
+		c, err := imapclient.DialStartTLS(addr, &imapclient.Options{
+			TLSConfig: &tls.Config{ServerName: host},
+		})
+		if err != nil {
+			return nil, fmt.Errorf("imap connect starttls: %w", err)
+		}
+		return c, nil
+	case "none":
+		c, err := imapclient.DialInsecure(addr, nil)
+		if err != nil {
+			return nil, fmt.Errorf("imap connect plain: %w", err)
+		}
+		return c, nil
+	default:
+		return nil, fmt.Errorf("imap: unknown tls mode %q", tlsMode)
+	}
+}
+
+// ListFolders retrieves all mailbox folders and detects junk/trash folders.
+func ListFolders(c *imapclient.Client) ([]FolderInfo, error) {
+	listCmd := c.List("", "*", nil)
+	mailboxes, err := listCmd.Collect()
+	if err != nil {
+		return nil, fmt.Errorf("imap list folders: %w", err)
+	}
+
+	var folders []FolderInfo
+	for _, mb := range mailboxes {
+		fi := FolderInfo{Name: mb.Mailbox}
+
+		// Check special-use attributes (RFC 6154)
+		for _, attr := range mb.Attrs {
+			if attr == imapv2.MailboxAttrJunk {
+				fi.Excluded = true
+				fi.Reason = "special_use"
+				break
+			}
+			if attr == imapv2.MailboxAttrTrash {
+				fi.Excluded = true
+				fi.Reason = "special_use"
+				break
+			}
+		}
+
+		// Fallback: case-insensitive name matching
+		if !fi.Excluded {
+			lower := strings.ToLower(mb.Mailbox)
+			for _, jt := range junkTrashNames {
+				if lower == jt {
+					fi.Excluded = true
+					fi.Reason = "name_match"
+					break
+				}
+			}
+		}
+
+		folders = append(folders, fi)
+	}
+
+	return folders, nil
+}