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>

CHANGELOG.md

@@ -7,6 +7,14 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ### Added
 
+- A page per delivery (`/deliveries/{id}`), reached from the *Details* link on
+  every send-log row. It carries what the log itself no longer shows — the
+  sending domain, the application the message was submitted under, the Postfix
+  queue id to search the system log for, when the status was last reported —
+  so the journal can grow fields without the table having to find columns for
+  them. *Back* returns to the page and filters the row was opened from, rebuilt
+  from the log's own parameters only.
+
 - A **DNS** badge in the domain list, one per row, carrying the same
   ok/warn/error/unknown vocabulary as the rest of the panel: the worst of that
   domain's DKIM, SPF and DMARC checks, so a domain whose records were never
@@ -44,6 +52,17 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ### Changed
 
+- The delivery log lists what identifies a message and nothing else: time,
+  sender, recipient, subject, status. Domain and application, which were a
+  column each, remain the log's two filters and now appear per message on the
+  delivery page. The two dropped columns were the widest thing in the table
+  after the addresses, and both repeat down the page whenever a filter is set.
+- Subjects are decoded for display as well as on the way in, so the rows the
+  journal-milter recorded before it decoded them itself — the ones an operator
+  is most likely to still be reading — show their text rather than
+  `=?utf-8?Q?…?=`. The decoder moved to `internal/mailhdr` and is shared by the
+  milter and the panel; it is idempotent, so a row decoded once passes through
+  unchanged.
 - The panel's navigation is a column down the left edge instead of a bar across
   the top. As a bar it did not fit on one row — six page entries and the
   session block against the panel's width — and had to be split into two,