feat(ui/monitoring): redesign Parked Messages page with filters, drawer, and bulk actions

Triage was painful on the old layout: a lone Site dropdown sat on a sparse
row, errors were truncated mid-sentence with a per-row View/Hide toggle
that on expand pushed an unwrapped <pre> through the table and shoved the
Actions column off-screen, all rows looked the same regardless of age or
attempt count, and OriginInstance — which tells you which instance
produced the failure — wasn't displayed at all even though the data was
on the entity.

This pass:

- Adds a real filter bar: Site, Category, Target system, Origin instance,
  Age window, free-text search. Category/Target/Origin/Age/Search filter
  the loaded page client-side; Site still drives the server query (and
  changing site now auto-queries — one fewer click).
- Replaces the in-table expansion with an Offcanvas detail drawer.
  Clicking a row slides in a side panel with full message ID + copy,
  category label, origin, attempts, both timestamps in relative + absolute
  form, the complete error (pre-wrap, scrollable), and big Retry / Discard
  buttons. The table never overflows.
- Stacks Target + Method into one column (target in semibold, method
  small/muted below) and surfaces Origin as a code-styled chip in a new
  column ("—" muted when null).
- Severity left-border on each row, derived client-side from
  AttemptCount/MaxAttempts and age of the last attempt: red when retries
  are exhausted and last attempt was in the past hour, amber when
  exhausted but stale, muted grey otherwise.
- Mini attempt progress bar under the n/max count, red when fully
  exhausted and amber while partial.
- Relative timestamps ("5m ago", "1h ago", "2d ago") with absolute UTC on
  hover via the title attribute — applies in both the table and the drawer.
- Bulk select: header checkbox selects the filtered set, per-row
  checkboxes. When ≥1 selected, a sticky action strip slides in below the
  filter bar offering Retry selected / Discard selected with the usual
  confirm dialog. Toast reports per-item success/failure counts.
- Summary line next to the title: "N parked · K target systems · oldest
  Xh ago" (and "(showing M of N)" when filters are active).
- ParkedMessageEntry contract extended additively with MaxAttempts,
  Category, and OriginInstance so the UI has the data it needs for
  severity, the category filter, and the new column.
- Bumped page size from 25 to 50 to better match the dense layout.

src/ScadaLink.StoreAndForward/ParkedMessageHandlerActor.cs

@@ -44,7 +44,10 @@ public class ParkedMessageHandlerActor : ReceiveActor
                             ErrorMessage: m.LastError ?? string.Empty,
                             AttemptCount: m.RetryCount,
                             OriginalTimestamp: m.CreatedAt,
-                            LastAttemptTimestamp: m.LastAttemptAt ?? m.CreatedAt))
+                            LastAttemptTimestamp: m.LastAttemptAt ?? m.CreatedAt,
+                            MaxAttempts: m.MaxRetries,
+                            Category: m.Category,
+                            OriginInstance: m.OriginInstanceName))
                         .ToList();
 
                     return new ParkedMessageQueryResponse(