Add a black theme for AMOLED screens (#181) (#399)

Settings → Appearance → Theme gets a fourth option, Black, next to System, Light and Dark. It's the dark theme with pure black backgrounds, and it works with Material You as well as Calendula's own palette.

- The month cells and the hour grid are `surfaceContainer` tiles on a `surface` ground, so making every container black would erase the grid lines. floret-kit's `toPureBlack()` sets the ground to #000 and scales the containers down to about #13, #1A, #25 and #30. The tiles stay a shade above black, and sheets and menus still stand out from the page.
- Black always means dark. It's an option in the theme picker, so it doesn't follow the system's light/dark setting.
- Event colours read `isSystemInDarkTheme()` in 15 places, so an in-app Dark or Black on a phone in light mode got light-theme tints. They now read `LocalFloretDarkTheme`, the theme's own decision. The bug also affected Dark, so the changelog has a Fixed entry for it.
- The window backdrop came from the night resource qualifier, so an in-app theme flashed the system's light or dark background before the first frame. `WindowThemeCache` keeps a SharedPreferences copy of the theme choice, and `onCreate` reads it to set the backdrop and bar icons. The copy is written when the stored setting loads, so the first cold start after updating still shows the old backdrop.
- Widgets keep their current themes. They get their theme work in the next milestone.

Needs jlmakiola/floret-kit#13, which is merged. The submodule is pinned to its merge commit on floret-kit `main` (`b1905da`).

Tested with new unit tests for the transform and for saving and reloading `BLACK`, and by hand on the emulator.

Closes #181

Co-authored-by: Jean-Luc Makiola <business@jeanlucmakiola.de>
Reviewed-on: https://codeberg.org/jlmakiola/calendula/pulls/399

CHANGELOG.md

@@ -13,8 +13,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **Tap a phone number in an event to call it.** Phone numbers in an event's title, location and description are now links. Tapping one opens your phone app with the number filled in, ready to call. A conference dial-in like "+1 646 558 8656,,850123#" brings its meeting ID along. Meeting IDs and PINs on their own, dates, times and amounts stay plain text. Long-pressing a field copies it, even when you press on a link ([#339]).
 - **Formatted event descriptions read properly.** Descriptions from Outlook, Exchange and Google Calendar are often stored as HTML, which used to show up as a wall of tags. The event view now shows them as formatted text: bold, italics, lists and links work, table rows sit on one line each, and the fonts and colours the sender picked are left out so the text matches the app. Search matches the text you see. The edit form still shows the original, so saving an event never changes its formatting ([#271]).
 - **Scroll back to earlier days in the agenda.** Scroll up past today to see earlier days' events, loaded a month at a time. Past days follow the past-events setting: Dim fades them, and with Hide the agenda still starts at today ([#367]).
+- **A black theme for AMOLED screens.** Settings → Appearance → Theme now has Black next to System, Light and Dark. It's the dark theme with pure black backgrounds, which saves battery on AMOLED screens and makes event colours stand out more. Day cells and the hour grid stay a shade above black so the grid lines still show, and sheets and menus still stand out from the page. It works with Material You colours too. The home-screen widgets aren't affected ([#181]).
 
 ### Fixed
+- **Event colours follow the theme you picked in Calendula.** With the app set to Dark on a phone in light mode, or the other way round, events took their colours from the phone's setting, so they could come out too pale or too dark. They now match the app's theme. Starting the app with a theme set in Calendula also no longer flashes the phone's light or dark background first ([#181]).
 - **Events on the day the clocks change show at their real time.** In the day, 3-day and week views, events later on the day of a daylight saving change were drawn an hour off, so a 14:45 event showed at 15:45. They now sit at their own time. When the clocks go back, 02:00 to 03:00 happens twice: an event in that hour now stays in the right one when you move, resize or edit it, and the editor and event details add the zone, like CEST or CET, so you can tell the two apart ([#387]).
 - **The agenda always starts today.** Coming from the month or day view, the agenda used to start at the day you had open there, so "Next 30 days" could show the 1st to the 30th of the month. It now opens on today with your chosen range. Backing out of a day you opened from the agenda still returns you to where you were ([#367]).
 - **Multi-day events dim on the days already behind you.** With past events dimmed, an event running over several days stayed at full strength on its earlier days until it ended. Those days now dim in the agenda, month, week and day views ([#367]).
@@ -1867,3 +1869,4 @@ automatically, with zero telemetry and no internet permission.
 [#339]: https://codeberg.org/jlmakiola/calendula/issues/339
 [#387]: https://codeberg.org/jlmakiola/calendula/issues/387
 [#271]: https://codeberg.org/jlmakiola/calendula/issues/271
+[#181]: https://codeberg.org/jlmakiola/calendula/issues/181