fix(shell-web): Step 3 stop-hook — pass full canvas width to host

Codex stop-hook flagged: "web smoke paints only the toolbar".

Root cause: shell-web's `paint_inspector` still passed the Step
1b leftover `280.0` to `host.paint`, but Step 3's WidgetHost
layout takes ~1/4 width per rail. With viewport_width=280 the
rail_w computation:

    rail_w = ((280.0 / 4.0) - 8.0).min(240.0).max(0.0) = 62.0

falls below MIN_RAIL_WIDTH (80), so the host's early-return
silently fired and only the toolbar painted. The smoke HTML
canvas is 960×640 — the host was getting a synthetic
viewport that didn't reflect reality.

Fix: pass `960.0` to `host.paint`, matching the smoke HTML's
`<canvas id="op" width="960">`. Now LayerPanel + CanvasViewport
+ PropertyPanel all paint into the canvas.

Inline comment cites the codex finding so a future hardcoded
viewport width regression is obvious.

Bundle untouched at 624 474 bytes gzip / 0 env.* imports.

crates/openpencil-shell-web/src/lib.rs

@@ -118,8 +118,17 @@ impl Inner {
             },
             Color::WHITE,
         );
-        // Inspector slice: 280 px wide column on the left.
-        self.host.paint(&mut self.backend, 280.0);
+        // Step 3: pass the FULL canvas width (matches the smoke
+        // HTML's `<canvas id="op" width="960">`). The host's
+        // `paint(&self, backend, viewport_width)` lays out
+        // Toolbar-top + LayerPanel-left + CanvasViewport-center
+        // + PropertyPanel-right; viewport_width below the
+        // MIN_RAIL_WIDTH cutoff would silently skip everything
+        // except the toolbar — codex stop-hook
+        // "web smoke paints only the toolbar" caught this when
+        // the leftover Step 1b 280-px hardcode was still in
+        // place.
+        self.host.paint(&mut self.backend, 960.0);
         self.backend.end_frame();
         if let Some(err) = self.backend.take_present_error() {
             return Err(err);