fix(plot-style): load plot style tables written by other applications

Tables copied from other applications into the plot styles folder were
listed but refused on selection, showing as "missing" with the reason
lost on a console: the loader failed hard on a checksum or a text length
that did not add up and on any non-UTF-8 byte — and those files carry
their descriptions in Windows-1252.

- decompress_ctb reads the file as it is: a checksum or length mismatch is
  reported and read past (a genuinely cut-off file is still an error), a
  zlib stream with a bad trailer is retried as raw deflate, and text that
  is not UTF-8 is read as Windows-1252 (pens and colours are numbers and
  unaffected; other code pages only show wrong characters in descriptions).
- PlotStyleTable carries load_warnings; every load site reports them on the
  command line, so a table that needed tolerance says so.
- load_named explains itself: a .stb name says named tables are not
  supported yet, a .ctb missing from the folder says which folder to copy
  it into (instead of "Unknown built-in plot style").
- The Plot dialog shows the reason under the Table row (style_error) —
  from a selection, from a page setup that names a table, and from the
  Load… picker, whose error used to be swallowed.
- A table picked with Load… from outside the plot styles folder is copied
  into it (unless a file of that name is already there), so it is listed
  from then on and page setups naming it resolve after a restart.

Tests: flipped checksum, text length off by one, a short payload field,
Windows-1252 descriptions, a truncated stream, .stb and missing names.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ui/window/plot.rs

@@ -431,6 +431,9 @@ pub struct PlotDialogState {
     /// The selected setup references a style table that is not loaded.
     #[serde(skip)]
     pub style_missing: bool,
+    /// Why the selected table could not be loaded, shown under the Table row.
+    #[serde(skip)]
+    pub style_error: Option<String>,
     /// Named page setups in the document (refreshed when the dialog opens).
     #[serde(skip)]
     pub page_setups: Vec<String>,
@@ -489,6 +492,7 @@ impl Default for PlotDialogState {
             show_plot_styles: false,
             plot_styles: Vec::new(),
             style_missing: false,
+            style_error: None,
             page_setups: Vec::new(),
             selected_setup: String::new(),
             name_input: None,
@@ -670,6 +674,17 @@ fn check_enabled<'a>(
         .into()
 }
 
+/// Why the selected plot style table is not loaded, under the Table row —
+/// the file's own problem, or where a copy would be found.
+fn style_note<'a>(s: &'a PlotDialogState) -> Element<'a, Message> {
+    match &s.style_error {
+        Some(error) if s.style_missing => {
+            text(error.clone()).size(10).style(muted_style).into()
+        }
+        _ => Space::new().height(0).into(),
+    }
+}
+
 fn panel<'a>(content: impl Into<Element<'a, Message>>) -> Element<'a, Message> {
     container(content)
         .width(Length::Fill)
@@ -1241,6 +1256,7 @@ pub fn view_window(
             PlotDlgMsg::Style,
             width,
         ),
+        style_note(s),
         check_enabled(
             t!("Plot with plot styles"),
             s.apply_plot_styles,