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/io/mod.rs

@@ -1522,7 +1522,12 @@ mod save_failure_tests {
 // ── Plot Style Table ──────────────────────────────────────────────────────
 
 /// Show a file-open dialog and load the selected CTB or STB file.
-pub async fn pick_plot_style() -> Option<plot_style::PlotStyleTable> {
+/// Pick a plot style table file and load it. `Ok(None)` when the picker was
+/// cancelled; `Err` says why the chosen file could not be read. A table
+/// picked from outside the plot styles folder is copied into it (unless a
+/// file of that name is already there), so the dialog lists it from now on
+/// and page setups naming it resolve after a restart.
+pub async fn pick_plot_style() -> Result<Option<plot_style::PlotStyleTable>, String> {
     let dialog = crate::sys::file_dialog()
         .set_title(crate::t!("Load Plot Style Table").as_ref())
         .add_filter(crate::t!("Plot Style Tables").as_ref(), &["ctb", "CTB"])
@@ -1533,8 +1538,26 @@ pub async fn pick_plot_style() -> Option<plot_style::PlotStyleTable> {
         Ok(dir) => dialog.set_directory(dir),
         Err(_) => dialog,
     };
-    let handle = dialog.pick_file().await?;
-    plot_style::PlotStyleTable::load(&crate::sys::handle_path(&handle)).ok()
+    let Some(handle) = dialog.pick_file().await else {
+        return Ok(None);
+    };
+    let path = crate::sys::handle_path(&handle);
+    let table = plot_style::PlotStyleTable::load(&path)?;
+    #[cfg(not(target_arch = "wasm32"))]
+    if let (Ok(dir), Some(file_name)) = (plot_style::ensure_plot_styles_dir(), path.file_name()) {
+        let destination = dir.join(file_name);
+        let same_place = path.parent().is_some_and(|parent| {
+            parent
+                .canonicalize()
+                .ok()
+                .zip(dir.canonicalize().ok())
+                .is_some_and(|(a, b)| a == b)
+        });
+        if !same_place && !destination.exists() {
+            let _ = std::fs::copy(&path, &destination);
+        }
+    }
+    Ok(Some(table))
 }
 
 // ── Image file picker ─────────────────────────────────────────────────────