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.

NOTICE

@@ -0,0 +1,58 @@
+# Third-party notices
+
+ihasmail is licensed under the AGPL-3.0; see LICENSE. This file records work by
+other people that ships inside it and the terms it comes under.
+
+## Colour palettes
+
+Four of the palettes offered in Settings › Appearance are the work of their own
+projects and are used under the MIT licence. Only the published colour values
+are used — no code, and nothing from anyone else's reimplementation of them.
+The values as fetched from each project are recorded in
+`.palette-sources/palettes-upstream.md`, and the shades between them are
+derived by `scripts/build-palettes.py`, which also lifts any tier that would
+not meet the contrast ihasmail claims.
+
+### Dracula and Alucard
+
+Copyright (c) 2016 Dracula Theme — https://github.com/dracula/dracula-theme
+Licensed under the MIT licence. "Dracula" is the dark variant and "Alucard" the
+light one; both are published in that repository's own "Color Palette (OSS)"
+section.
+
+### Gruvbox
+
+Copyright (c) 2018 Pavel Pertsev — https://github.com/morhetz/gruvbox
+Licensed under the MIT licence.
+
+### Rosé Pine
+
+Copyright (c) 2021 Rosé Pine — https://github.com/rose-pine/rose-pine-theme
+Licensed under the MIT licence. The light variant is "Dawn".
+
+### Tokyo Night
+
+Copyright (c) 2019 enkia — https://github.com/enkia/tokyo-night-vscode-theme
+Licensed under the MIT licence. The light variant is "Day".
+
+---
+
+The MIT licence, under which all four are used:
+
+    Permission is hereby granted, free of charge, to any person obtaining a
+    copy of this software and associated documentation files (the "Software"),
+    to deal in the Software without restriction, including without limitation
+    the rights to use, copy, modify, merge, publish, distribute, sublicense,
+    and/or sell copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+    DEALINGS IN THE SOFTWARE.