Let the reader set the clock to a date, and run it backwards

The clock from #33 could only run forwards from now. The Display panel's
clock now has a Date (UTC) field, a native datetime-local in a form, so Enter
submits it and the browser holds it to its min and max. It jumps the clock to
that date, and the clock carries on from there at the rate it was running at.
A Backwards toggle (aria-pressed) runs the same four rates the other way. The
radios still pick the rate's size and keep the direction when it changes.

The window is AD 1 to AD 3000. The end is where Standish's Table 2 stops
being fitted (3000 BC to AD 3000; every planet within 0.29 degrees of
Horizons at each date measured out to 3000). The start is the date input's
own floor. TimeStore.setDate refuses anything outside it, and NaN, and leaves
the clock where it was. The field is read as UTC. Its dates are proleptic
Gregorian, as a Date is, so before 1582 they run up to ten days ahead of the
Julian-calendar dates history gives. The window is written beside
CLOCK_WINDOW, with the moons' shorter reach (Phobos 11 degrees out by 2100).

The system note now names the date it is drawn for, to the minute:
"... to 2020-12-21 18:00 UTC.", or "to now, <date> UTC." at the present.

Measured in the app (port 4311, keyboard only: fill, Enter):
- Set to 2020-12-21 18:00 UTC, Jupiter and Saturn seen from Earth's drawn
  position are 0.113 degrees apart. Horizons gives 0.102 geocentric
  (geometric 0.1017, astrometric 0.1018). Distances: 5.9267 and 10.8296 AU
  against Horizons' 5.9258 and 10.8270.
- 3001-06-01 is refused by the form (validity false) and the clock does not
  move.
- Backwards at 1 d/s: -2.011 days in about 2 s.
- The field's accessible name is "Date (UTC)" and its description is the
  window. Its colour-scheme is dark, so the picker icon shows on the HUD.
- Back to now puts the field back on the present too.

Unit suite 799 -> 805: two tests for the store, three for the dock, one for
the scene note. Eleven guarded mutants; each changed its file and made its
named test fail. Among them: the window check dropped, the wall clock not
re-anchored on a jump, the radio dropping the direction, the field read as
local time, and the note not naming the date.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/app/features/galaxy-system/galaxy-system-scene.component.ts

@@ -1763,7 +1763,10 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
       // Where the orbits come from, and for the Sun how far from the present they hold: each
       // body's card names its own source and epoch.
       const source = this.bodies.some((body) => body.systemStarId === star.id) ? 'JPL mean elements, the planets’ fit for 3000 BC to AD 3000, and the SBDB’s osculating ones for Ceres, Eris, Haumea and Makemake,' : 'published elements';
-      this.hudNote.set(`Orbits propagated from ${source} to ${this.time.atNow() ? 'the current date' : 'the date on the clock'}.`);
+      // Named to the minute, in UTC like the date field: a jump to 18:00 on a given day is a
+      // question about that hour, and the note is where the answer says which sky it is.
+      const drawnFor = `${this.time.date().toISOString().slice(0, 16).replace('T', ' ')} UTC`;
+      this.hudNote.set(`Orbits propagated from ${source} to ${this.time.atNow() ? 'now, ' : ''}${drawnFor}.`);
       this.hudRange.set(
         formatAu(
           this.engine.visibleHalfHeight(