Fix per-job queue ETA showing for jobs that cannot start now

The scheduler only writes waiting_reason on the model-based assignment
path, so a job pinned to a specific printer sits behind a running print
with no marker at all. Every such job rendered an identical "starts now"
ETA that was wrong by the length of everything ahead of it.

Decide eligibility on the page instead: an item gets an ETA only when its
printer is idle and it is the item the scheduler would dispatch next,
following the same ordering the scheduler uses. Staged and future-
scheduled items do not block the item behind them, matching the
scheduler, and items conditional on a previous print are excluded.

The value also froze at first render, since react-query's structural
sharing keeps the queue reference stable and nothing re-rendered the row.
formatETA now accepts a base instant and the page drives it from a 30s
clock shared by every visible row.

Retire the borrowed printers.estimatedCompletion tooltip for a queue key
that says what the number means, translated into all 13 locales.

frontend/src/utils/date.ts

@@ -319,14 +319,20 @@ export function formatTimeOnly(
  * @param remainingMinutes - Minutes until completion
  * @param timeFormat - Time format setting ('system', '12h', '24h')
  * @param t - Optional i18n translation function
+ * @param baseTime - Instant to count from, in epoch ms. Defaults to the current
+ *   clock. Callers that render an ETA for something not yet started must pass a
+ *   value that changes over time, or the string freezes at first render: it is
+ *   only recomputed when the component re-renders, which does not happen while
+ *   the underlying data is unchanged (#2740).
  * @returns Formatted ETA string (e.g., "3:45 PM", "Tomorrow 9:30 AM", "Wed 2:00 PM")
  */
 export function formatETA(
   remainingMinutes: number,
   timeFormat: TimeFormat = 'system',
-  t?: (key: string) => string
+  t?: (key: string) => string,
+  baseTime?: number
 ): string {
-  const now = new Date();
+  const now = baseTime != null ? new Date(baseTime) : new Date();
   const eta = new Date(now.getTime() + remainingMinutes * 60 * 1000);
 
   const today = new Date(now);