Fix proposeDimensions error: skip call when terminal not ready

When terminal is not ready after max attempts, go directly to fallback
dimensions instead of falling through to call proposeDimensions() which
throws 'viewport.scrollBarWidth' TypeError.

Root cause: FitAddon.proposeDimensions() checks _renderService.dimensions
before accessing viewport.scrollBarWidth, but dimensions can be valid
while viewport is still undefined during terminal initialization.

src/textual_webterm/static/js/terminal.ts

@@ -360,7 +360,13 @@ class WebTerminal {
               window.requestAnimationFrame(() => attemptFitAndResize(attempt + 1));
               return;
             }
-            // Fall through to use fallback dimensions
+            // Terminal not ready after max attempts - use fallback directly
+            // Don't call proposeDimensions() as it will throw
+            console.warn("Terminal not ready after max attempts, using fallback dimensions");
+            this.terminal.resize(fallback.cols, fallback.rows);
+            this.resizeState.lastValidSize = fallback;
+            this.send(["resize", { width: fallback.cols, height: fallback.rows }]);
+            return;
           }
           
           const dims = (() => {