fix(print): complete Windows printer discovery, honor the true default, surface failures; make CTB loading usable (#1261)

Printer enumeration used EnumPrinters level 4 only, which is served
from the local registry cache and misses Win11 per-user network
printers; an empty result now retries with level 2 (spooler RPC) and
merges the resolved default. The dialog's 'System default printer'
used to delegate to the shell print verb — the default PDF handler's
own printer choice, not the system default; it now resolves
GetDefaultPrinterW and dispatches printto naming that printer
explicitly. Every dispatch probes OpenPrinterW first, so a printer
that vanished between listing and printing produces a clear error
instead of ShellExecute's silent nothing, and all SE_ERR_* codes carry
readable reasons.

Legacy CTB: picking an unparseable table used to swallow the failure
(.ok()) — the user saw nothing at all. Load errors now reach the
command line, the picker accepts .stb alongside .ctb (the container
gains the STBVER1 header; .stb application errors state the actual
reason), and a table loaded from anywhere is copied into the user's
plot-styles folder so it persists in the dialog, matching AutoCAD.
Buffer parsing for both EnumPrinters layouts is unit-tested; the
missing-printer path was exercised against the real spooler (existence
probe only, no job).

src/app/update/mod.rs

@@ -8340,7 +8340,7 @@ impl OpenCADStudio {
             Message::PlotStyleLoad => {
                 Task::perform(crate::io::pick_plot_style(), Message::PlotStyleLoaded)
             }
-            Message::PlotStyleLoaded(Some(table)) => {
+            Message::PlotStyleLoaded(Ok(Some(table))) => {
                 if table.is_stb {
                     self.command_line.push_error(
                         crate::t!(
@@ -8368,7 +8368,14 @@ impl OpenCADStudio {
                 self.plot_dialog.plot_styles = crate::io::plot_style::available_ctb_names();
                 Task::none()
             }
-            Message::PlotStyleLoaded(None) => Task::none(),
+            // The user closed the file dialog without picking anything.
+            Message::PlotStyleLoaded(Ok(None)) => Task::none(),
+            // A picked table that failed to parse used to vanish silently
+            // (`.ok()` collapsed it into "cancelled"); surface it instead.
+            Message::PlotStyleLoaded(Err(error)) => {
+                self.command_line.push_error(&error);
+                Task::none()
+            }
             Message::PlotStyleClear => {
                 self.active_plot_style = None;
                 self.plot_dialog.style_name.clear();