feat: Windows build and Android groundwork (Issues #48, #42)

Shared groundwork:
- paths.rs: folders for data, settings and downloads per system (Linux XDG
  unchanged, Windows %APPDATA%/%LOCALAPPDATA%, Android app folder)
- Linux-only parts behind cfg: Wayland on-screen keyboard (osk/wayland.rs),
  display backend and app id, the vendored winit touch extensions
- run_with(event_loop) so that other entry points can start the app

Windows (#48):
- cross-compiled with MinGW into a portable ink-notes_<version>_x86_64.exe
  (system DLLs only), no console window in release builds
- file picker: user folders and drives; held notebook locks are detected
  (ERROR_LOCK_VIOLATION)
- updater: replaces the running .exe via rename, active at the next start
- octotablet panicking without the Tablet PC components no longer crashes
  the app
- release: .exe built in CI and part of the signed checksums
- all library tests pass under wine

Android (#42):
- android_main entry point, winit with android-native-activity,
  android_logger; octotablet and arboard left out there
- cargo check and the cdylib build for aarch64-linux-android pass; the window
  surface does not follow the activity lifecycle yet

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.lock

@@ -85,6 +85,24 @@ version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fc7eb209b1518d6bb87b283c20095f5228ecda460da70b44f0802523dea6da04"
 
+[[package]]
+name = "android_log-sys"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "84521a3cf562bc62942e294181d9eef17eb38ceb8c68677bc49f144e4c3d4f8d"
+
+[[package]]
+name = "android_logger"
+version = "0.13.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c494134f746c14dc653a35a4ea5aca24ac368529da5370ecf41fe0341c35772f"
+dependencies = [
+ "android_log-sys",
+ "env_logger",
+ "log",
+ "once_cell",
+]
+
 [[package]]
 name = "android_system_properties"
 version = "0.1.6"
@@ -699,6 +717,15 @@ dependencies = [
  "syn 2.0.119",
 ]
 
+[[package]]
+name = "env_logger"
+version = "0.10.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4cd405aab171cb85d6735e5c8d9db038c17d3ca007a4d2c25f337935c3d90580"
+dependencies = [
+ "log",
+]
+
 [[package]]
 name = "epaint"
 version = "0.27.2"
@@ -1414,6 +1441,7 @@ name = "ink-notes"
 version = "0.38.0"
 dependencies = [
  "ab_glyph",
+ "android_logger",
  "arboard",
  "base64",
  "bytemuck",
@@ -1426,6 +1454,7 @@ dependencies = [
  "hayro",
  "hayro-svg",
  "image",
+ "log",
  "minisign-verify",
  "naga",
  "octotablet",