fix(tz): server-side timezone conversion for all user-facing time displays

The audit showed only 5 of ~11 time-display surfaces converted to the
user's timezone; everything else rendered pgx-decoded UTC. The server
owns conversion — clients (web/app/KOReader) receive display-ready
strings or machine-readable instants and never convert.

- Progress page: LastUpdated formatted in the requesting user's tz
  (was raw UTC).
- Analytics: date-range defaults, day buckets, most-active-day, device
  last-sync, and popular-book last-read all computed in the user's tz —
  evening sessions no longer land on the next UTC day. Also fixed the
  end-date boundary: the query bounds with <= and the end bound was
  midnight, so the end date's own sessions (including 'today') never
  appeared.
- Queue page: new created_at_display field (admin tz) rendered by the
  template; created_at stays RFC3339 for API consumers.
- Admin hash-conflicts SSR bridge: detected date rendered in the
  admin's tz.
- OPDS: .UTC() before the Z-suffixed feed timestamps (honesty fix —
  correct today only because the session zone happens to be UTC).
- KOReader annotation datetimes deliberately untouched (device owns its
  display).

internal/utils/timeformat.go

@@ -0,0 +1,31 @@
+package utils
+
+import (
+	"time"
+)
+
+// UserLocation resolves an IANA timezone name to a *time.Location, falling
+// back to UTC for empty or invalid values. The single resolution point for
+// server-side display conversion: the server owns timezone rendering, so
+// clients receive display-ready strings (or tz-aware instants) and never
+// convert themselves.
+func UserLocation(timezone string) *time.Location {
+	if timezone == "" {
+		return time.UTC
+	}
+	loc, err := time.LoadLocation(timezone)
+	if err != nil || loc == nil {
+		return time.UTC
+	}
+	return loc
+}
+
+// FormatTimeInTimezone renders t in the given IANA timezone as
+// MM-DD-YYYY HH:MM AM/PM (the established display format across the
+// templates). Zero times render empty.
+func FormatTimeInTimezone(t time.Time, timezone string) string {
+	if t.IsZero() {
+		return ""
+	}
+	return t.In(UserLocation(timezone)).Format("01-02-2006 03:04 PM")
+}