docs: add v1.2.3 release notes and update README

Document Kotlin migration, i18n, and UI changes; align JAR examples with 1.2.3.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: code4teaching

README.md

@@ -8,7 +8,7 @@
 
 [![Build](https://github.com/code4teaching/VisuStruct/actions/workflows/maven-build.yml/badge.svg?branch=master)](https://github.com/code4teaching/VisuStruct/actions)
 
-**VisuStruct** is a **structure-chart editor** for **Nassi–Shneiderman** diagrams (desktop, Java / Swing).
+**VisuStruct** is a **structure-chart editor** for **Nassi–Shneiderman** diagrams (desktop, Kotlin / Swing).
 
 Under **Settings → Languages** the **user interface** can be set to any of **21 languages** aligned with **VisuStruct-swift** (English, German, Spanish, French, Italian, Polish, Turkish, Korean, Japanese, Greek, Ukrainian, Arabic, Russian, Hebrew, Dutch, Swedish, Danish, Norwegian Bokmål, Chinese Simplified/Traditional, European Portuguese). Stored as `uilanguage` in `visustruct.properties` (e.g. `zh_Hans`, `pt_PT`). UI strings are synced from the Swift app where keys overlap: `python3 tools/sync_java_messages_from_swift.py` (map: `VisuStruct-swift/tools/java_swift_i18n_map.json`). Desktop-only labels may still fall back to English.
 
@@ -22,6 +22,14 @@ git clone https://github.com/code4teaching/VisuStruct.git
 
 ---
 
+## Changes in 1.2.3 (summary)
+
+- **Kotlin:** full migration of production code; Java reference under `src/main/java-reference/`; tests still Java.
+- **i18n:** localized element editor and dialogs; `tools/resync_message_bundles.py` when Swift repo is absent.
+- **UI:** one-touch expandable element-editor split.
+
+Full notes: [`release-notes/v1.2.3.md`](release-notes/v1.2.3.md).
+
 ## Changes in 1.2.2 (summary)
 
 - **Languages:** **21 UI languages** (VisuStruct-swift); `tools/sync_java_messages_from_swift.py`.
@@ -148,7 +156,7 @@ This release aligns **branding and technical identity** with **VisuStruct**. **`
 - **Java 17** target (build and run with JDK 17+).
 - **FlatLaf** light/dark themes; **JDOM2**; no legacy AppleJavaExtensions.
 - **Motif** look-and-feel removed; **Metal** and FlatLaf available.
-- Current fat JAR name follows **`pom.xml`** **`version`** (e.g. **`visustruct-1.2.2.jar`**).
+- Current fat JAR name follows **`pom.xml`** **`version`** (e.g. **`visustruct-1.2.3.jar`**).
 
 ---
 
@@ -223,13 +231,13 @@ chmod +x mvnw
 Output (version from `pom.xml`):
 
 ```text
-target/visustruct-1.2.2.jar
+target/visustruct-1.2.3.jar
 ```
 
 ## Run
 
 ```bash
-java -jar target/visustruct-1.2.2.jar
+java -jar target/visustruct-1.2.3.jar
 ```
 
 Double-click may work if `.jar` is associated with Java.

release-notes/v1.2.3.md

@@ -0,0 +1,26 @@
+# VisuStruct 1.2.3
+
+Release **after** **1.2.2**.
+
+## Kotlin migration
+
+- **Production sources** are now **Kotlin** under `src/main/kotlin/` (UI, control, structure elements, code generation, simulation XML).
+- Former Java sources are kept for reference only in **`src/main/java-reference/`** (not compiled).
+- **Tests** remain **Java** under `src/test/java/` and run against the Kotlin main code.
+- **Behaviour and file format** unchanged: **`.visustruct` / XML** projects remain compatible.
+
+## Internationalization
+
+- **Element editor** and common **dialogs** (zoom, colour, font, element edit) use bundle keys (`editor.*`, `dialog.*`) with **EN + DE** and EN fallback in all **21** UI locales.
+- **`tools/resync_message_bundles.py`** rebuilds locale files from `Messages_en.properties` when **VisuStruct-swift** is not available locally.
+- **`tools/translation_aliases.json`** maps new keys from existing translated Java keys where possible.
+- Full Swift parity still requires a sibling **VisuStruct-swift** clone and `python3 tools/sync_java_messages_from_swift.py`.
+
+## UI
+
+- **Element editor** split pane: **one-touch expandable** divider (`editorSplit`).
+
+## Build
+
+- Maven: **`de.visustruct:visustruct:1.2.3`**
+- Fat JAR: **`visustruct-1.2.3.jar`** (`./mvnw package` → `target/`)