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>

packaging/flatpak/cargo-sources.json

@@ -116,6 +116,32 @@
         "dest": "cargo/vendor/android-properties-0.2.2",
         "dest-filename": ".cargo-checksum.json"
     },
+    {
+        "type": "archive",
+        "archive-type": "tar-gzip",
+        "url": "https://static.crates.io/crates/android_log-sys/android_log-sys-0.3.2.crate",
+        "sha256": "84521a3cf562bc62942e294181d9eef17eb38ceb8c68677bc49f144e4c3d4f8d",
+        "dest": "cargo/vendor/android_log-sys-0.3.2"
+    },
+    {
+        "type": "inline",
+        "contents": "{\"package\": \"84521a3cf562bc62942e294181d9eef17eb38ceb8c68677bc49f144e4c3d4f8d\", \"files\": {}}",
+        "dest": "cargo/vendor/android_log-sys-0.3.2",
+        "dest-filename": ".cargo-checksum.json"
+    },
+    {
+        "type": "archive",
+        "archive-type": "tar-gzip",
+        "url": "https://static.crates.io/crates/android_logger/android_logger-0.13.3.crate",
+        "sha256": "c494134f746c14dc653a35a4ea5aca24ac368529da5370ecf41fe0341c35772f",
+        "dest": "cargo/vendor/android_logger-0.13.3"
+    },
+    {
+        "type": "inline",
+        "contents": "{\"package\": \"c494134f746c14dc653a35a4ea5aca24ac368529da5370ecf41fe0341c35772f\", \"files\": {}}",
+        "dest": "cargo/vendor/android_logger-0.13.3",
+        "dest-filename": ".cargo-checksum.json"
+    },
     {
         "type": "archive",
         "archive-type": "tar-gzip",
@@ -1013,6 +1039,19 @@
         "dest": "cargo/vendor/enum_dispatch-0.3.13",
         "dest-filename": ".cargo-checksum.json"
     },
+    {
+        "type": "archive",
+        "archive-type": "tar-gzip",
+        "url": "https://static.crates.io/crates/env_logger/env_logger-0.10.2.crate",
+        "sha256": "4cd405aab171cb85d6735e5c8d9db038c17d3ca007a4d2c25f337935c3d90580",
+        "dest": "cargo/vendor/env_logger-0.10.2"
+    },
+    {
+        "type": "inline",
+        "contents": "{\"package\": \"4cd405aab171cb85d6735e5c8d9db038c17d3ca007a4d2c25f337935c3d90580\", \"files\": {}}",
+        "dest": "cargo/vendor/env_logger-0.10.2",
+        "dest-filename": ".cargo-checksum.json"
+    },
     {
         "type": "archive",
         "archive-type": "tar-gzip",