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/generate-appconfig.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+
+# Generates apple/VniDrop/Generated/AppConfig.swift from the shared app.properties
+# so app-wide constants (privacy policy URL, …) have a single source of truth
+# across Apple and KMP. Regenerate instead of editing the output.
+
+set -euo pipefail
+
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+repo_root="$(cd "$script_dir/../.." && pwd)"
+config_file="${VNIDROP_APP_PROPERTIES:-$repo_root/app.properties}"
+output_dir="${VNIDROP_APPLE_GENERATED_DIR:-$repo_root/apple/VniDrop/Generated}"
+
+read_property() {
+	local key=$1
+	local value
+	value="$(sed -n "s/^${key}=//p" "$config_file")"
+	[[ -n "$value" ]] || { printf 'Missing %s in %s\n' "$key" "$config_file" >&2; exit 1; }
+	[[ $(printf '%s\n' "$value" | wc -l | tr -d ' ') == 1 ]] ||
+		{ printf 'Duplicate %s in %s\n' "$key" "$config_file" >&2; exit 1; }
+	printf '%s' "$value"
+}
+
+# Escape for a Swift string literal.
+swift_escape() {
+	printf '%s' "$1" | sed -e 's/\\/\\\\/g' -e 's/"/\\"/g'
+}
+
+privacy_url="$(read_property PRIVACY_POLICY_URL)"
+
+mkdir -p "$output_dir"
+tmp="$(mktemp "$output_dir/.AppConfig.swift.XXXXXX")"
+cat > "$tmp" <<EOF
+// Generated by apple/scripts/generate-appconfig.sh from app.properties.
+// Regenerate this file instead of editing it.
+
+import Foundation
+
+/// App-wide constants injected at build time from the shared \`app.properties\`.
+enum AppConfig {
+	static let privacyPolicyURL = URL(string: "$(swift_escape "$privacy_url")")!
+}
+EOF
+mv "$tmp" "$output_dir/AppConfig.swift"