Fix four things the Options survey turned up

**TEXTFILL marked the drawing modified.** It is an application preference —
it lives in a process global and is snapshotted into settings.json — but it
was missing from the list of names that persist, so setting it dirtied the
open drawing instead and never reached the settings file through that path.

**LTSCALE and CELTSCALE validated differently depending on the door.** The
standalone commands reject a value of zero or less, because a non-positive
linetype scale degenerates every dash pattern in the drawing. `SETVAR LTSCALE
-5` accepted it. A test drives both entry points for both variables and fails
on the SETVAR path without the fix.

**ISAVEBAK, SAVETIME and FILEASSOC were missing from the autocomplete
registry** although all three dispatch, so the command line never offered
them.

**`bg_color` and `paper_bg_color` were dead fields**, always written as `None`
and ignored on load since the model-space configuration superseded them.
Removing them is safe: the config is `#[serde(default)]` and ignores unknown
keys, so existing settings files simply stop carrying them.

One thing the survey flagged that turned out not to be a bug: on the Display
page the model background row hides itself outside Custom mode while the
paper and desk rows do not. That asymmetry is correct — the canvas mode
governs only the model background, and the other two apply in every mode.

Co-Authored-By: Claude Opus 5 <[email protected]>

src/app/update/file.rs

@@ -435,8 +435,6 @@ impl OpenCADStudio {
             pick_add: self.pick_add,
             pick_drag_rect: self.pick_drag_rect,
             quick_properties: self.quick_properties,
-            bg_color: None,
-            paper_bg_color: None,
             language: self.language,
             cliprompt_lines: crate::app::settings::clamp_clipromptlines(self.cliprompt_lines),
             commandline_fade_ms: crate::app::settings::clamp_commandline_fade_ms(
@@ -516,7 +514,6 @@ impl OpenCADStudio {
         self.pick_add = s.pick_add;
         self.pick_drag_rect = s.pick_drag_rect;
         self.quick_properties = s.quick_properties;
-        // Legacy settings.bg_color / paper_bg_color are superseded by model_space config.
         if crate::i18n::set_language(s.language).is_ok() {
             self.language = s.language;
         }