Draw core-time from floret-kit via submodule + composite build

Phase 0 of the shared-kit plan: floret-kit is embedded as a git submodule and
wired in with Gradle includeBuild("floret-kit"), so the app depends on
de.jeanlucmakiola.floret:core-time and Gradle substitutes the local source
module (built from source — F-Droid-safe). Proves the submodule + composite-build
pipeline before any design layers move.

- Remove app copies of DayWindow + Instant date/time formatting (now in
  core-time); repoint imports to de.jeanlucmakiola.floret.time.
- DateTimeField gains an explicit import (was same-package).
- CI checkouts fetch submodules recursively.
- Clean composite build + unit tests + debug assemble green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "floret-kit"]
+	path = floret-kit
+	url = https://gitea.jeanlucmakiola.de/makiolaj/floret-kit.git