Six palettes, each with a light half and a dark one

The theme was one enum -- system, light, dark, ihasmail -- where one value
carried a whole palette and implied dark. That works for exactly one
palette. The two questions now come apart: which palette, and which side.

Classic is the plain light and dark this app has always had. ihasmail's
own palette gains a day version, so the background of the dark one becomes
the text of the light one and the two read as one palette from either end.
Dracula, Gruvbox, Rosé Pine and Tokyo Night are the work of their own
projects, used under the MIT licence, and taken from each project's own
repository rather than from anyone's reimplementation. What was fetched is
recorded in .palette-sources/ and credited in NOTICE.

Giving ihasmail's palette a light half removed a whole special case.
Nothing is one-sided any more, so a palette can no longer override the
mode, the toggle no longer has to set a palette aside on the way to light
and remember it, and the greyed-out control that explained all that is
gone. The old lastDarkTheme, which existed only for that, is gone with it.

The shades between the published colours are derived rather than guessed:
these projects publish twelve to twenty values and ihasmail needs about
thirty. scripts/build-palettes.py computes the tiers and then measures
every text colour against the surface it sits on -- 4.5:1 for prose, 3:1
for borders and marks -- lifting anything short towards white on a dark
ground and towards black on a light one, so a lifted tier keeps its hue.
It refuses to write a palette that would not pass.

Every one of the nine halves needed at least one lift. These palettes are
built for code editors, not for prose at this size: Dracula's comment grey
is 3.03:1 on its own background and Rosé Pine's gold is 2.7:1 on Dawn.
Shipping them as published would have quietly ended the WCAG AA claim.

Two things caught while checking rather than while writing. The generated
blocks were appended to the end of the stylesheet, which put them after
the accent variants at equal specificity -- so choosing an accent over one
of the new palettes did nothing at all. They now sit before those rules,
where the existing ihasmail block's own comment says they have to. And
that block was unqualified, so it would have shadowed the new light half;
it is now explicitly the dark one.

Settings written before this carry `theme` and no palette, and are read
through the old enum. `theme` is still written back, derived, because a
device on an older build reads it and would otherwise be stranded on a
theme nobody chose.

web/src/views/AppShell.tsx

@@ -3,7 +3,8 @@ import { Link, useLocation } from "wouter";
 import { BookOpen, Calendar, ChevronsUpDown, FolderOpen, Globe, HelpCircle, LogOut, Mail, Menu as MenuIcon, Moon, PenSquare, Plus, RefreshCw, Settings, Sun, Upload, Users, X } from "lucide-react";
 import { useSession } from "@/store/session";
 import { withBase } from "@/lib/basePath";
-import { toggleTarget, useEffectiveTheme, useSettings } from "@/store/settings";
+import { useEffectiveTheme, useSettings } from "@/store/settings";
+import { toggleTarget } from "@/lib/palette";
 import { useMail } from "@/store/mail";
 import { draftFromMailto, useCompose } from "@/store/compose";
 import { Avatar, useIsMobile } from "@/ui/misc";
@@ -268,18 +269,20 @@ function QuotaBar() {
  */
 function ThemeToggle() {
   const effective = useEffectiveTheme();
-  const lastDarkTheme = useSettings((s) => s.settings.lastDarkTheme);
+  const settings = useSettings((s) => s.settings);
   const update = useSettings((s) => s.update);
-  const next = toggleTarget(effective, lastDarkTheme);
-  // The label names where you are going, and going back is not always "dark"
-  // any more -- it is whichever theme you were on before flipping to light.
-  const label = next === "light" ? "light mode" : next === "system" ? "your system theme" : next === "ihasmail" ? "the ihasmail theme" : "dark mode";
+  const prefersDark = Boolean(window.matchMedia?.("(prefers-color-scheme: dark)").matches);
+  const next = toggleTarget({ palette: settings.palette, mode: settings.mode }, prefersDark);
+  // Name where it is going, and by the palette when the palette is changing --
+  // going back to ihasmail's own colours is not the same as "dark mode".
+  // The palette never changes now, so the label is only ever the side.
+  const label = next.mode === "light" ? t("light mode") : t("dark mode");
   return (
     <button
       className="icon-btn"
       aria-label={t("Switch to {theme}", { theme: label })}
       title={t("Switch to {theme}", { theme: label })}
-      onClick={() => update({ theme: next })}
+      onClick={() => update(next)}
     >
       {effective === "dark" ? <Sun size={21} /> : <Moon size={21} />}
     </button>