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-tests/integration/layout/position.test.tsx

@@ -71,7 +71,7 @@ test("relative position offsets visual position while keeping flow", async () =>
     )),
     { columns: 100 },
   );
-  expect(lastFrame({ trimLines: true })).toBe(" BA  ");
+  expect(lastFrame({ trimLines: true })).toBe(" BA");
 });
 
 test("static position ignores offsets", async () => {

packages/runtime-tests/integration/layout/width-height.test.tsx

@@ -30,7 +30,7 @@ test("set width in percent", async () => {
     )),
     { columns: 100 },
   );
-  expect(lastFrame({ trimLines: true })).toBe("A    B    ");
+  expect(lastFrame({ trimLines: true })).toBe("A    B");
 });
 
 test("set min width", async () => {