fix: trim trailing whitespace per line in paint output to match Ink behavior

Ink's Output.get() calls .trimEnd() on each line of the rendered grid.
Without this, vue-tui's paint grid always fills the full root width with
spaces, causing raw frame output to contain 100-char-wide padding. This
broke tests using { raw: true } and made meaningful trailing-space
assertions impossible with { trimLines: true }.

Also corrects test assertions for 'set width in percent' and 'relative
position' that expected trailing spaces which Ink never produces.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/runtime/src/paint/paint.ts

@@ -49,7 +49,7 @@ class Output {
         placeLine(grid, op.x, op.y + lineIdx, line);
       }
     }
-    return grid.map((row) => row.join("")).join("\n");
+    return grid.map((row) => row.join("").trimEnd()).join("\n");
   }
 }