refactor: make grid visibility per-drawing, not a global setting

Grid on/off now round-trips through the drawing's *Active VPort, so it is a
per-file view setting like every other CAD app treats it. Drop show_grid
from UserSettings (and its settings.txt key) and instead adopt the opened
file's saved grid_on on load. A legacy "grid=" line in an old settings file
is harmlessly ignored.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/settings.rs

@@ -65,7 +65,6 @@ pub struct UserSettings {
     pub ortho: bool,
     pub polar: bool,
     pub polar_increment_deg: f32,
-    pub show_grid: bool,
     pub snap_enabled: bool,
     pub otrack: bool,
     /// Active snap modes, in `SNAP_ORDER`.
@@ -90,7 +89,6 @@ impl Default for UserSettings {
             ortho: false,
             polar: false,
             polar_increment_deg: 45.0,
-            show_grid: false,
             snap_enabled: false,
             otrack: false,
             snap_modes: vec![
@@ -135,7 +133,6 @@ impl UserSettings {
                         s.polar_increment_deg = v;
                     }
                 }
-                "grid" => s.show_grid = val == "1",
                 "osnap" => s.snap_enabled = val == "1",
                 "otrack" => s.otrack = val == "1",
                 "default_assoc_prompted" => s.default_assoc_prompted = val == "1",
@@ -180,12 +177,11 @@ impl UserSettings {
             .collect::<Vec<_>>()
             .join(",");
         let body = format!(
-            "dyn={}\northo={}\npolar={}\npolar_increment_deg={}\ngrid={}\nosnap={}\notrack={}\ndefault_assoc_prompted={}\nsnap_modes={}\ndisabled_plugins={}\nplugin_repos={}\n",
+            "dyn={}\northo={}\npolar={}\npolar_increment_deg={}\nosnap={}\notrack={}\ndefault_assoc_prompted={}\nsnap_modes={}\ndisabled_plugins={}\nplugin_repos={}\n",
             b(self.dyn_input),
             b(self.ortho),
             b(self.polar),
             self.polar_increment_deg,
-            b(self.show_grid),
             b(self.snap_enabled),
             b(self.otrack),
             b(self.default_assoc_prompted),

src/app/update.rs

@@ -185,7 +185,6 @@ impl OpenCADStudio {
             ortho: self.ortho_mode,
             polar: self.polar_mode,
             polar_increment_deg: self.polar_increment_deg,
-            show_grid: self.show_grid,
             snap_enabled: self.snapper.snap_enabled,
             otrack: self.snapper.otrack_enabled,
             snap_modes: super::settings::UserSettings::modes_from(self.snapper.enabled.iter()),
@@ -205,7 +204,6 @@ impl OpenCADStudio {
         self.ortho_mode = s.ortho;
         self.polar_mode = s.polar;
         self.polar_increment_deg = s.polar_increment_deg;
-        self.show_grid = s.show_grid;
         self.snapper.snap_enabled = s.snap_enabled;
         self.snapper.otrack_enabled = s.otrack;
         self.snapper.enabled = s.snap_modes.iter().copied().collect();
@@ -600,6 +598,11 @@ impl OpenCADStudio {
                 // selected — see #21.
                 self.sync_ribbon_from_selection();
                 self.tabs[i].scene.restore_saved_camera();
+                // Grid visibility is a per-drawing view setting — adopt the
+                // opened file's saved state rather than a global preference.
+                if let Some(grid_on) = self.tabs[i].scene.active_vport_grid_on() {
+                    self.show_grid = grid_on;
+                }
                 self.sync_render_mode_to_active_tile(i);
                 self.tabs[i].last_synced_camera_gen = self.tabs[i].scene.camera_generation;
                 self.tabs[i].dirty = false;

src/scene/mod.rs

@@ -5639,6 +5639,17 @@ impl Scene {
         }
     }
 
+    /// Grid visibility stored on the `*Active` VPort entry, applied to the app's
+    /// grid toggle when a file loads (grid is a per-drawing view setting, not a
+    /// global preference). `None` when there is no active entry. (#121)
+    pub fn active_vport_grid_on(&self) -> Option<bool> {
+        self.document
+            .vports
+            .iter()
+            .find(|v| v.name == "*Active")
+            .map(|v| v.grid_on)
+    }
+
     /// Set the paper-space camera from the sheet viewport's stored view.
     /// Returns true if a valid sheet viewport was found and the camera was set.
     ///