feat(panel): move a delivery's details onto its own page

The delivery log now lists what identifies a message and nothing else —
time, sender, recipient, subject, status — and links each row to
/deliveries/{id}, which carries the rest: the sending domain, the
application it was submitted under, the Postfix queue id to search the
system log for, and when the status was last reported. Domain and
application were a column each; they were the widest thing in the table
after the addresses and repeat down every filtered page, and they remain
the log's two filters. Back returns to the page and filters the row was
opened from, rebuilt from the log's own parameters only.

Subjects are now decoded for display as well as on the way in. The milter
has decoded them since 8add005, but the rows it wrote before that still
hold the raw =?utf-8?Q?...?= header, and those are the ones an operator is
most likely to still be reading. The decoder moves to internal/mailhdr,
shared by the milter and the panel; it is idempotent, so a row decoded
once passes through unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/web/handlers_monitor.go

@@ -4,9 +4,11 @@ import (
 	"errors"
 	"io/fs"
 	"net/http"
+	"net/url"
 	"strconv"
 
 	"github.com/mixeme/selfpost/internal/logtail"
+	"github.com/mixeme/selfpost/internal/mailhdr"
 	"github.com/mixeme/selfpost/internal/postfix"
 	"github.com/mixeme/selfpost/internal/store"
 )
@@ -50,6 +52,58 @@ func (s *Server) handleDeliveriesRows(w http.ResponseWriter, r *http.Request) {
 	s.renderFragment(w, http.StatusOK, "deliveries_rows", data)
 }
 
+// handleDelivery renders one send-log row in full. The log itself carries only
+// what identifies a message at a glance — when, who to and from, what about,
+// how it ended — and every remaining field (domain, application, queue id, when
+// the status was last reported) lives here, one page per row, so widening the
+// journal never costs the table a column.
+func (s *Server) handleDelivery(w http.ResponseWriter, r *http.Request) {
+	id, err := strconv.ParseInt(r.PathValue("id"), 10, 64)
+	if err != nil || id <= 0 {
+		http.NotFound(w, r)
+		return
+	}
+	row, err := s.store.GetSendLog(id)
+	if err != nil {
+		// A row pruned on the retention window is gone, not broken.
+		if errors.Is(err, store.ErrSendLogNotFound) {
+			http.NotFound(w, r)
+			return
+		}
+		logf("panel: delivery %d: %v", id, err)
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+	row.Subject = mailhdr.DecodeSubject(row.Subject)
+	s.render(w, http.StatusOK, "delivery", map[string]any{
+		"Title":  "SelfPost — delivery",
+		"User":   currentUser(r),
+		"Active": "deliveries",
+		"Row":    row,
+		// Where the row came from, so "Back" returns to the page and filters
+		// the operator was looking at rather than the top of an unfiltered log.
+		"BackURL": deliveriesBackURL(r),
+	})
+}
+
+// deliveriesBackURL rebuilds the delivery-log URL a detail page was opened
+// from. Only the log's own parameters are carried over, and each is re-encoded
+// by url.Values, so nothing a visitor appends to the link can travel back into
+// the page as markup or as a different destination.
+func deliveriesBackURL(r *http.Request) string {
+	q := r.URL.Query()
+	back := url.Values{}
+	for _, k := range []string{"domain", "app", "p"} {
+		if v := q.Get(k); v != "" {
+			back.Set(k, v)
+		}
+	}
+	if len(back) == 0 {
+		return "/deliveries"
+	}
+	return "/deliveries?" + back.Encode()
+}
+
 // sendLogData reads the domain/app filters and page number off the query
 // string, queries the store, and assembles everything the template needs
 // (filter dropdown options plus the current selection, rows, and pagination).
@@ -69,6 +123,12 @@ func (s *Server) sendLogData(r *http.Request) (map[string]any, error) {
 	if err != nil {
 		return nil, err
 	}
+	// Decode on the way out as well as on the way in: rows the journal-milter
+	// wrote before it decoded subjects itself still hold the raw header, and
+	// they are the ones an operator is most likely to be looking at.
+	for i := range rows {
+		rows[i].Subject = mailhdr.DecodeSubject(rows[i].Subject)
+	}
 	domains, err := s.store.ListDomains()
 	if err != nil {
 		return nil, err