feat(packaging): open files from argv, ship Windows MSI installer (#36)

Cross-platform prerequisite for OS-level file associations: the boot
task now consumes `std::env::args_os().nth(1)` as a path and dispatches
`Message::OpenRecent` for it. Flag-style args (starting with `-`) are
ignored, and `OpenRecent` already does the file-existence check, so a
bogus path lands as a clean command-line error instead of a panic.

Linux: the AppImage's desktop entry adds `image/vnd.dwg` alongside the
existing `image/vnd.dxf` MIME type and grows a `%f` placeholder in
`Exec=` so the launcher forwards the picked file path. Once the
AppImage is integrated with the system (xdg-mime / AppImageLauncher),
double-clicking a .dwg / .dxf opens it in Open CAD Studio.

Windows: new `packaging/windows/main.wxs` defines an MSI installer —
per-machine install to `Program Files\Open CAD Studio`, Start Menu
shortcut, `MajorUpgrade` so newer MSIs replace older ones, and a
single ProgID `OpenCADStudio.Drawing` that owns both `.dwg` and
`.dxf` and launches the exe with the file as `argv[1]`. The icon is
pulled from the SVG logo, converted to multi-resolution ICO at build
time with the runner's pre-installed ImageMagick. CI now also runs
`candle` / `light` (WiX Toolset 3, pre-installed on `windows-latest`)
and uploads both the bare `.exe` (portable) and `.msi` (installer)
to the release.

macOS Info.plist already declares the DWG / DXF UTIs; combined with
the argv-handling change above, double-clicking a drawing in Finder
will open it in Open CAD Studio without further packaging changes.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/mod.rs

@@ -1036,7 +1036,22 @@ impl OpenCADStudio {
             Message::UpdateCheckResult,
         );
         let focus_cmd = s.focus_cmd_input();
-        (s, Task::batch([open_main, check_update, focus_cmd]))
+        // Open a file path passed on the command line. Used by the
+        // OS file association: double-clicking a .dwg in the file
+        // explorer launches the binary with the file path as the
+        // first positional argument. Unknown / flag-style args are
+        // ignored. `Message::OpenRecent` does the existence check
+        // and reports a clean error if the path is bogus.
+        let cli_open: Task<Message> = std::env::args_os()
+            .nth(1)
+            .map(PathBuf::from)
+            .filter(|p| !p.as_os_str().to_string_lossy().starts_with('-'))
+            .map(|p| Task::done(Message::OpenRecent(p)))
+            .unwrap_or_else(Task::none);
+        (
+            s,
+            Task::batch([open_main, check_update, focus_cmd, cli_open]),
+        )
     }
 }