Build with Gradle in two modules, and free core of JavaFX

Etappen 1 and 2 of the Compose migration (43 commits).

The Maven build, the single source tree and the JPMS module-info went together.
core now holds the domain and app-desktop the user interface, both built by
Gradle, packaged through jpackage from Gradle, with SpotBugs and PMD wired in.
JavaFX moves to the classpath, which is why the launcher is Main and not the
Application subclass. The PR check, the nightly artifacts, the release and the
AUR packages all build from Gradle; the macOS DMG points at its output.
Verified on Linux, macOS and Windows.

core then loses JavaFX entirely. It observes through kst4contest.observe --
ObservableValue, ObservableRoster, and UiDispatcher as the one boundary a
worker thread crosses to touch the interface. The controller collections become
rosters and the visible station list a pure derivation rather than a
FilteredList/SortedList chain, which is also what fixed two focus defects only
a load test had exposed: the keyboard focus stays where the operator put it,
and the send field keeps it after a row click.

The rule that outlives this commit: ObservableRoster is the store and the list
a view is bound to is a mirror of it, never the other way round.

packaging/icons/generate-icons.sh

@@ -0,0 +1,90 @@
+#!/usr/bin/env bash
+# Regenerate every raster icon from kst4contest.svg.
+#
+# The rasters are committed, so nothing in CI runs this — it exists so the
+# icons can be rebuilt reproducibly whenever the SVG changes. Requires
+# rsvg-convert (librsvg) and Python Pillow.
+#
+#   packaging/icons/generate-icons.sh
+#
+# Outputs, all regenerated in place and mirrored into src/main/resources/icons
+# so the in-app window icon matches what the launcher shows:
+#   kst4contest.png   256px, the size jpackage --icon feeds to every Linux
+#                     package and the size hicolor/256x256/apps expects
+#   kst4contest.ico   16/24/32/48/64 as BMP, 128/256 as PNG (Windows convention)
+#   kst4contest.icns  ic07-ic14, the modern macOS set up to 1024px
+
+set -euo pipefail
+
+ICON_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+REPO_ROOT="$(cd "${ICON_DIR}/../.." && pwd)"
+cd "$REPO_ROOT"
+
+for tool in rsvg-convert python3; do
+    command -v "$tool" >/dev/null || { echo "Missing: $tool" >&2; exit 1; }
+done
+python3 -c "import PIL" 2>/dev/null || { echo "Missing: Python Pillow" >&2; exit 1; }
+
+# The SVG asks for 'Space Grotesk' and falls back to Helvetica Neue, then Arial.
+# Whichever is installed is baked into the raster, so a machine with a different
+# font set produces visibly different icons. Warn rather than fail — Arial is
+# the author's own third choice and renders acceptably.
+if ! fc-list 2>/dev/null | grep -qi "space grotesk"; then
+    echo "WARNING: 'Space Grotesk' is not installed; the KST wordmark will be"
+    echo "         rendered with a fallback font (checked: $(fc-match 'Space Grotesk' 2>/dev/null | head -1))."
+fi
+
+python3 - <<'PY'
+import struct, subprocess, os
+from PIL import Image
+
+SRC, OUT = "packaging/icons/kst4contest.svg", "packaging/icons"
+_cache = {}
+
+def render(size):
+    if size not in _cache:
+        p = f"/tmp/kst4contest-icon-{size}.png"
+        subprocess.run(["rsvg-convert", "-w", str(size), "-h", str(size), SRC, "-o", p], check=True)
+        _cache[size] = p
+    return _cache[size]
+
+# --- Linux -------------------------------------------------------------
+Image.open(render(256)).convert("RGBA").save(f"{OUT}/kst4contest.png", optimize=True)
+
+# --- Windows -----------------------------------------------------------
+def bmp_entry(im):
+    w, h = im.size
+    px = im.load()
+    hdr = struct.pack("<IiiHHIIiiII", 40, w, h * 2, 1, 32, 0, 0, 0, 0, 0, 0)
+    rows = [bytes(bytearray().join(
+        bytes((px[x, y][2], px[x, y][1], px[x, y][0], px[x, y][3])) for x in range(w)
+    )) for y in range(h - 1, -1, -1)]
+    mask = bytes(((w + 31) // 32) * 4 * h)
+    return hdr + b"".join(rows) + mask
+
+entries = []
+for size in [16, 24, 32, 48, 64]:
+    entries.append((size, bmp_entry(Image.open(render(size)).convert("RGBA"))))
+for size in [128, 256]:
+    entries.append((size, open(render(size), "rb").read()))
+
+offset = 6 + 16 * len(entries)
+dir_, blobs = b"", b""
+for size, data in entries:
+    dim = 0 if size == 256 else size
+    dir_ += struct.pack("<BBBBHHII", dim, dim, 0, 0, 1, 32, len(data), offset)
+    blobs += data
+    offset += len(data)
+open(f"{OUT}/kst4contest.ico", "wb").write(struct.pack("<HHH", 0, 1, len(entries)) + dir_ + blobs)
+
+# --- macOS -------------------------------------------------------------
+chunks = b""
+for t, size in [("ic11", 32), ("ic12", 64), ("ic07", 128), ("ic13", 256),
+                ("ic08", 256), ("ic14", 512), ("ic09", 512), ("ic10", 1024)]:
+    data = open(render(size), "rb").read()
+    chunks += t.encode("ascii") + struct.pack(">I", len(data) + 8) + data
+open(f"{OUT}/kst4contest.icns", "wb").write(b"icns" + struct.pack(">I", len(chunks) + 8) + chunks)
+PY
+
+cp packaging/icons/kst4contest.{png,ico,icns} src/main/resources/icons/
+echo "Regenerated packaging/icons and src/main/resources/icons from kst4contest.svg"