refactor(theme,motion): draw identity (theme + content transitions) from floret-kit

Adopt floret-kit's identity module, shared with Agendula:

- CalendulaTheme becomes a thin wrapper over the kit's FloretExpressiveTheme
  (dynamic colour, system light/dark, standard motion scheme). Calendula keeps
  its identity locally: the seed-derived CalendulaLightFallback/DarkFallback and
  CalendulaTypography. No call-site changes — CalendulaTheme keeps its name.
- The generic content transitions move to identity: calendarExpandEnter ->
  expandEnter, calendarCollapseExit -> collapseExit, calendarItemEnter ->
  itemEnter, calendarAnimateItem -> animateItemMotion, calendarFadeThrough ->
  fadeThrough, plus Modifier.predictiveBack and rememberReduceMotion (same
  names). Call sites across the screens repointed to floret.identity.
- CalendarTransitions.kt keeps only what's genuinely calendar-specific: the
  directional month/week/day slide (calendarSlideTransition) and its specs
  (rememberCalendarSlideSpec / rememberCalendarFadeSpec).

Compile + unit tests + lintDebug green.

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

app/build.gradle.kts

@@ -163,6 +163,7 @@ dependencies {
     implementation("de.jeanlucmakiola.floret:core-locale")
     implementation("de.jeanlucmakiola.floret:core-crash")
     implementation("de.jeanlucmakiola.floret:core-reminders")
+    implementation("de.jeanlucmakiola.floret:identity")
     implementation(libs.kotlinx.coroutines.core)
 
     debugImplementation(libs.androidx.ui.tooling)