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and ignored on load since the model-space configuration superseded them.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 <noreply@anthropic.com>

src/app/commands/mod.rs

@@ -554,6 +554,11 @@ inventory::submit!(crate::command::CommandRegistration {
         "GRIPHOT",
         "GRIPHOVER",
         "GRIPOBJLIMIT",
+        // Dispatched all along, but absent from the registry, so command-line
+        // completion never offered them.
+        "ISAVEBAK",
+        "SAVETIME",
+        "FILEASSOC",
         // Reset selected entities' overrides to follow their layer.
         "SETBYLAYER",
         // Remove duplicate objects; set drawing base point; audit integrity;