feat(config): shared app.properties for app-wide constants

Add a single source of truth (root app.properties) for public app-wide
constants, injected at build time on both platforms instead of hardcoding.

- Apple: generate-appconfig.sh -> Generated/AppConfig.swift (wired into
  `make apple-app-config`), consumed as AppConfig.privacyPolicyURL.
- KMP: generateAppConfig task -> AppConfig.kt (mirrors DiagnosticsBuildConfig),
  consumed as AppConfig.PRIVACY_POLICY_URL.

Replaces the stale hardcoded privacy-policy URL on both sides with
https://vnidrop.sudosy.fr/privacy/.

Also fix the Apple release core build: disable release LTO in build-core.sh
(Cargo forbids lto in a build-override) to avoid the proc-macro
"mis-aligned LINKEDIT string pool" corruption, so release archives are
compact instead of shipping the debug core.

Update the app icon.

Tests: shell test for the generator (escaping, missing/duplicate key),
plus XCTest and jvmTest asserting the generated value matches app.properties.

apple/scripts/tests/test-generate-appconfig.sh

@@ -0,0 +1,59 @@
+#!/usr/bin/env bash
+
+# Tests apple/scripts/generate-appconfig.sh: the shared app.properties is read
+# correctly, values are emitted as valid escaped Swift, and a missing key fails.
+
+set -euo pipefail
+
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+generator="$script_dir/../generate-appconfig.sh"
+repo_root="$(cd "$script_dir/../../.." && pwd)"
+scratch="$(mktemp -d)"
+trap 'rm -rf "$scratch"' EXIT
+
+# Run the generator against a fixture app.properties, emitting into a temp dir.
+generate() {
+	VNIDROP_APP_PROPERTIES="$scratch/app.properties" \
+		VNIDROP_APPLE_GENERATED_DIR="$scratch/out" \
+		"$generator"
+}
+
+expect_failure() {
+	if "$@" >/dev/null 2>&1; then
+		printf 'Expected command to fail: %s\n' "$*" >&2
+		exit 1
+	fi
+}
+
+assert_contains() {
+	local file=$1 needle=$2
+	grep -qF "$needle" "$file" ||
+		{ printf 'Expected %s to contain: %s\n' "$file" "$needle" >&2; exit 1; }
+}
+
+out="$scratch/out/AppConfig.swift"
+
+# 1. Nominal value is emitted verbatim as a Swift URL literal.
+printf 'PRIVACY_POLICY_URL=%s\n' 'https://example.test/privacy/' > "$scratch/app.properties"
+generate
+assert_contains "$out" 'URL(string: "https://example.test/privacy/")!'
+assert_contains "$out" 'enum AppConfig'
+
+# 2. Characters special to a Swift string literal are escaped.
+printf 'PRIVACY_POLICY_URL=%s\n' 'https://a.test/"q"\z' > "$scratch/app.properties"
+generate
+assert_contains "$out" 'URL(string: "https://a.test/\"q\"\\z")!'
+
+# 3. A missing key fails instead of emitting an empty value.
+printf 'OTHER_KEY=value\n' > "$scratch/app.properties"
+expect_failure generate
+
+# 4. A duplicated key fails.
+printf 'PRIVACY_POLICY_URL=a\nPRIVACY_POLICY_URL=b\n' > "$scratch/app.properties"
+expect_failure generate
+
+# 5. The real committed app.properties produces an https URL.
+VNIDROP_APPLE_GENERATED_DIR="$scratch/real" "$generator"
+assert_contains "$scratch/real/AppConfig.swift" 'URL(string: "https://'
+
+printf 'generate-appconfig tests passed.\n'