release: cut v2.0.0 — write support complete

Version bumped to 2.0.0 / 13. No code changes beyond the version — 2.0.0
closes out Milestone 2 (write support, v1.1 through v2.0): the final slice
is the save-conflict dialog (external change → overwrite/discard, external
delete → informational close), plus the store refresh: descriptions and
README describe write support and reminders, and fastlane screenshots
(DE+EN, six each) ship for F-Droid. CHANGELOG [2.0.0] carries the details.

Quick-add was cut from scope (the prefilled form covers it); calendar
switching while editing moved to the v3 backlog. Both documented in the
roadmap.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/build.gradle.kts

@@ -23,8 +23,8 @@ android {
         applicationId = "de.jeanlucmakiola.calendula"
         minSdk = 29
         targetSdk = 36
-        versionCode = 12
-        versionName = "1.4.0"
+        versionCode = 13
+        versionName = "2.0.0"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }