Make the darker tone true-black AMOLED

The darker tone sat at #090908 for --background while --chrome was already
pure black, which both inverted the elevation ramp (frame strips darker than
the content beside them) and missed the point of an AMOLED tone — only a
true #000000 pixel is actually off on OLED.

--background now collapses into --chrome at pure black, so the main window,
connection manager and editor all render black end to end: panes, bars,
sidebar, gutter and textarea. Elevation is kept where it earns its place —
dialogs, popovers and toasts stay #181817, hover and selection #272726 — and
--border/--input go from 12%/15% to 16%/18% white, since on pure black those
lines are the only thing left defining panes, bars and rows.

No plumbing needed: buildPreload already bakes data-dark-tone onto every
window pre-paint, and tone changes broadcast live to open windows.

Co-Authored-By: Claude <noreply@anthropic.com>

docs/architecture.md

@@ -245,7 +245,10 @@ dark neutrals are a **warm gray family (hue 107)** anchored on `#1a1a19`
 and cool surfaces clash. Surface elevation ramp, both themes: `--chrome`
 (frame strips) sits *below* `--background` (content panes) < `--card`
 (popovers/dialogs) < `--accent` (hover). Dark ramp `#151514` → `#1a1a19` →
-`#20201f` → `#2e2e2d`; the "darker" tone drops `--chrome` to true black.
+`#20201f` → `#2e2e2d`. The "darker" tone is true-black AMOLED: `--chrome` AND
+`--background` are both `#000` (the frame/content step collapses on purpose),
+elevation survives only via `--card`/`--accent`, and `--border`/`--input` go to
+16%/18% white because those lines then carry the whole layout.
 `bg-chrome` is on the tab bar, status bar, transfer bar, the editor/manager
 header+footer strips and the manager's connection sidebar; panel-internal
 toolbars deliberately stay on `--background`. Accent tones: one