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/mod.rs

@@ -3290,7 +3290,8 @@ pub enum Message {
     /// Open file dialog to load a CTB/STB plot style table.
     PlotStyleLoad,
     /// Callback when the user picks (or cancels) a CTB/STB file.
-    PlotStyleLoaded(Option<crate::io::plot_style::PlotStyleTable>),
+    /// `Ok(None)` = cancelled; `Err` = the picked file failed to parse.
+    PlotStyleLoaded(Result<Option<crate::io::plot_style::PlotStyleTable>, String>),
     /// Clear the active plot style table.
     PlotStyleClear,
     /// Open/close the Plot Style panel.