test(web): finish the textarea-locator sweep after the architecture merge

preview-boot's hero wait was master's new textarea:enabled locator (the
conv refactor predates the Lexical composer); migrate it to the
data-composer-input surface like every other lane. Drop the unnecessary
DOMRect assertion oxlint flagged on the new Range stub.

apps/web/tests/assembled-boot.ts

@@ -164,7 +164,7 @@ export function installAssembledBootEnv(): void {
   if (typeof Range.prototype.getBoundingClientRect !== 'function') {
     Range.prototype.getBoundingClientRect = () => ({
       top: 0, bottom: 0, left: 0, right: 0, width: 0, height: 0, x: 0, y: 0, toJSON: () => ({}),
-    }) as DOMRect
+    })
   }
   beforeEach(() => {
     localStorage.clear()

apps/web/tests/preview-boot.e2e.ts

@@ -312,7 +312,7 @@ async function bootPreview(origin: string, browser: Browser): Promise<void> {
     const configureLater = page.getByRole('button', { name: 'Configure later' })
     await configureLater.waitFor({ timeout: 30_000 })
     await configureLater.click()
-    await page.locator('textarea:enabled[placeholder="Describe what you want to build"]')
+    await page.locator('[data-composer-input][data-placeholder="Describe what you want to build"]')
       .waitFor({ timeout: 30_000 })
 
     const exercised = await page.evaluate(async () => {