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/VniDrop/Features/Settings/SettingsSections.swift

@@ -375,7 +375,7 @@ struct StorageSettings: View {
 struct AboutSettings: View {
 	@ObservedObject var model: SettingsModel
 
-	private static let privacyPolicyURL = URL(string: "https://github.com/vnidrop/vnidrop")!
+	private static let privacyPolicyURL = AppConfig.privacyPolicyURL
 
 	var body: some View {
 		Section {