feat(cli): full command-line interface via clap

Replace the ad-hoc `--serve` arg sniff with a clap-parsed CLI:

- positional <file> to open at startup (also the OS file-association path)
- --new, --read-only, --backend, --safe-mode/--no-gpu
- --serve/--port (headless JSON automation, unchanged behaviour)
- --export IN OUT (headless format convert via the automation load/save infra)
- --script FILE (run command lines at startup)
- --log/RUST_LOG, plus --version/--help for free

GUI-only options pass to app::boot through a OnceLock config; read-only
gates the save paths. The web build skips parsing (no args).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/update.rs

@@ -947,6 +947,11 @@ impl OpenCADStudio {
             Message::ObjImportPath(None) => Task::none(),
 
             Message::SaveFile => {
+                if self.read_only {
+                    self.command_line
+                        .push_error("Read-only session (--read-only): saving is disabled.");
+                    return Task::none();
+                }
                 let i = self.active_tab;
                 // Stamp the live grid/snap toggles onto the VPort so the file
                 // reflects them even if they came from settings with no
@@ -973,6 +978,11 @@ impl OpenCADStudio {
             }
 
             Message::SaveAs => {
+                if self.read_only {
+                    self.command_line
+                        .push_error("Read-only session (--read-only): saving is disabled.");
+                    return Task::none();
+                }
                 let i = self.active_tab;
                 self.save_dialog_for_unsaved = false;
                 self.open_save_dialog_window(i)