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 d35b309, 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>

README.md

@@ -207,9 +207,13 @@ service healthy and will mail be accepted?"
   (level 2) are configured here. *Export domain* writes a single-domain archive;
   *Import a domain* on the Backup page reads one back in.
 - **Deliveries** (`/deliveries`) — searchable send log with server-side filters
-  by domain and application. Each row shows status `queued` (accepted, not yet
+  by domain and application. A row identifies its message and nothing more —
+  time, sender, recipient, subject and status `queued` (accepted, not yet
   delivered), `sent` (handed off successfully), or `rejected` (refused — for
-  example by a level-2 rate limit). Retention is controlled by
+  example by a level-2 rate limit); *Details* opens that row's own page
+  (`/deliveries/{id}`) with 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. Retention is controlled by
   `SEND_LOG_RETENTION_DAYS`.
 - **Mail queue** (`/mail-queue`) — live view of messages Postfix is still
   trying to deliver or deferring.