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>

version.properties

@@ -0,0 +1,22 @@
+# ============================================================
+# Meldestelle — Zentrale Versionsdatei (Semantic Versioning)
+# ============================================================
+# Format: MAJOR.MINOR.PATCH
+#   MAJOR — Breaking Changes / Inkompatible API-Änderungen
+#   MINOR — Neue Features (abwärtskompatibel)
+#   PATCH — Bugfixes (abwärtskompatibel)
+#
+# Diese Datei ist die Single Source of Truth für alle Versionen.
+# Gradle liest sie beim Build; CI/CD setzt Git-Tags basierend darauf.
+#
+# Workflow:
+#   1. Version hier erhöhen (z. B. 1.0.0 → 1.1.0)
+#   2. Commit + Push
+#   3. CI erstellt automatisch Git-Tag "v1.1.0"
+#   4. Release-Artefakte (.msi / .deb / .dmg) werden gebaut und hochgeladen
+# ============================================================
+VERSION_MAJOR=1
+VERSION_MINOR=0
+VERSION_PATCH=0
+# Qualifier: leer = Release, "SNAPSHOT" = Entwicklungsstand, "beta.1" = Vorversion
+VERSION_QUALIFIER=SNAPSHOT