yes.js: guard against the canvas not being in the DOM yet

The Rust-side NodeRef gate (previous commit) didn't actually close the
race on fast client-side re-navigation back to / - reproduced the
same crash again after that fix shipped. Harden the actual failure
point directly instead of chasing the exact Leptos/wasm-bindgen
timing: skip setup (not throw) if either canvas is missing.

yes.js

@@ -11,6 +11,22 @@ export class RasterizedYES {
     constructor() {
         this.rasterCanvas = document.getElementById('rasterCanvas');
         this.lineCanvas = document.getElementById('lineCanvas');
+
+        // The Rust side only constructs this once it's confirmed (via a
+        // NodeRef) that the canvas is mounted, but that guard has shown
+        // a real gap on fast client-side re-navigation back to `/` -
+        // this is the actual failure point, so it gets its own defense
+        // rather than depending on getting that timing exactly right
+        // from the other side of the wasm boundary. Leaving the
+        // instance otherwise-inert (no crash, no animation) rather than
+        // throwing mid-render - `stop()` already tolerates a partially
+        // (non-)initialized instance.
+        if (!this.rasterCanvas || !this.lineCanvas) {
+            console.warn('RasterizedYES: canvas not in DOM yet, skipping');
+            this.destroyed = true;
+            return;
+        }
+
         this.rasterCtx = this.rasterCanvas.getContext('2d');
         this.lineCtx = this.lineCanvas.getContext('2d');