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and ignored on load since the model-space configuration superseded them.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/settings.rs

@@ -269,12 +269,6 @@ pub struct UserSettings {
     pub pick_drag_rect: bool,
     /// Show the floating Quick Properties panel when objects are selected.
     pub quick_properties: bool,
-    /// Persisted viewport background colours (0–255 RGB); `None` = app default
-    /// (dark grey model / off-white paper). Applied to every drawing tab on
-    /// launch and to tabs opened later, so a chosen background survives restarts
-    /// (#188).
-    pub bg_color: Option<[u8; 3]>,
-    pub paper_bg_color: Option<[u8; 3]>,
     /// Interface language preference. `System` negotiates against the
     /// platform locale on every launch.
     pub language: crate::i18n::Language,
@@ -384,8 +378,6 @@ impl Default for UserSettings {
             pick_add: true,
             pick_drag_rect: false,
             quick_properties: false,
-            bg_color: None,
-            paper_bg_color: None,
             language: crate::i18n::Language::default(),
             cliprompt_lines: 3,
             commandline_fade_ms: 3000,