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/Tests/AppConfigTests.swift

@@ -0,0 +1,39 @@
+import XCTest
+@testable import VniDrop
+
+/// Verifies the build-time `AppConfig` (generated from the shared `app.properties`)
+/// exposes the expected, well-formed values to the app.
+final class AppConfigTests: XCTestCase {
+	func testPrivacyPolicyURLIsTheExpectedHTTPSEndpoint() {
+		let url = AppConfig.privacyPolicyURL
+		XCTAssertEqual(url.scheme, "https", "Privacy policy URL must be https")
+		XCTAssertEqual(url.absoluteString, "https://vnidrop.sudosy.fr/privacy/")
+	}
+
+	func testPrivacyPolicyURLMatchesTheSharedConfigFile() throws {
+		// Cross-check the generated constant against the single source of truth so a
+		// broken generator (or drift) is caught, not just a hardcoded copy.
+		let expected = try Self.privacyURLFromAppProperties()
+		XCTAssertEqual(AppConfig.privacyPolicyURL.absoluteString, expected)
+	}
+
+	/// Reads `PRIVACY_POLICY_URL` from the repo's `app.properties` by walking up
+	/// from this source file's location to the repository root.
+	private static func privacyURLFromAppProperties() throws -> String {
+		var dir = URL(fileURLWithPath: #filePath).deletingLastPathComponent()
+		for _ in 0..<8 {
+			let candidate = dir.appendingPathComponent("app.properties")
+			if FileManager.default.fileExists(atPath: candidate.path) {
+				let contents = try String(contentsOf: candidate, encoding: .utf8)
+				for line in contents.split(whereSeparator: \.isNewline) {
+					if line.hasPrefix("PRIVACY_POLICY_URL=") {
+						return String(line.dropFirst("PRIVACY_POLICY_URL=".count))
+					}
+				}
+				throw XCTSkip("PRIVACY_POLICY_URL missing in \(candidate.path)")
+			}
+			dir.deleteLastPathComponent()
+		}
+		throw XCTSkip("app.properties not found from \(#filePath)")
+	}
+}