feat(devops): configure desktop packaging and introduce semantic versioning

- Added `nativeDistributions` for Linux (.deb), Windows (.msi), and macOS (.dmg) in `build.gradle.kts` with platform-specific settings, embedded JRE, and JVM-args.
- Implemented centralized semantic versioning via `version.properties` as the single source of truth, applying it across all builds.
- Introduced CI/CD release workflow (`.gitea/workflows/release.yml`) for auto-tagging, artifact builds, and release summaries.
- Created `CHANGELOG.md` following Keep-a-Changelog format for tracking changes.
- Documented icon requirements and packaging steps in `ICONS_PLACEHOLDER.md`.
- Updated DevOps roadmap to reflect completed Sprint C-1 and C-2 tasks.

Signed-off-by: Stefan Mogeritsch <stefan.mo.co@gmail.com>

frontend/shells/meldestelle-desktop/src/jvmMain/resources/ICONS_PLACEHOLDER.md

@@ -0,0 +1,25 @@
+# App-Icons — Platzhalter
+
+Folgende Icon-Dateien müssen hier abgelegt werden, bevor ein Release-Build erstellt wird:
+
+| Datei       | Format | Größe        | Plattform      |
+|-------------|--------|--------------|----------------|
+| `icon.png`  | PNG    | 512×512 px   | Linux (.deb)   |
+| `icon.ico`  | ICO    | 256×256 px   | Windows (.msi) |
+| `icon.icns` | ICNS   | 1024×1024 px | macOS (.dmg)   |
+
+## Hinweise
+
+- PNG: RGBA, transparenter Hintergrund empfohlen
+- ICO: Multi-Size (16, 32, 48, 64, 128, 256 px) in einer Datei
+- ICNS: Mit `iconutil` (macOS) oder `png2icns` (Linux) aus PNG erzeugen
+
+## Schnell-Erzeugung (Linux, ImageMagick)
+
+```bash
+# PNG → ICO (Windows)
+convert icon.png -define icon:auto-resize=256,128,64,48,32,16 icon.ico
+
+# PNG → ICNS (macOS, benötigt libicns)
+png2icns icon.icns icon.png
+```