fix(web): reveal the caret when a persisted draft arrives after mount

ConversationSession seeds a stored draft in its own mount effect, and a parent's mount effect runs after its children's — so the unlock effect measured an empty mirror and never ran again for the draft that then appeared, leaving a restored long draft showing its head with the caret at its end. The effect now depends on the draft being non-empty; clearing on send and typing the first character flip it too, where both the focus and the reveal are no-ops. Pre-existing (the old geometry did not scroll for a programmatic value change either), fixed here because the reveal now exists.

Also from review: the golden's paste goes back to a block NOT ending in a newline, so the collapsed branch keeps a real engine under it while the standalone case owns the after-newline branch; the shared line-height rule names the reveal as its third consumer.

apps/web/tests/composer-draft-scroll.e2e.ts

@@ -452,7 +452,10 @@ describe('web e2e: composer draft scrolling', () => {
       const data = new DataTransfer()
       data.setData('text/plain', text)
       el.dispatchEvent(new ClipboardEvent('paste', { clipboardData: data, bubbles: true, cancelable: true }))
-    }, `\n${DRAFT}\n`)
+      // The ordinary shape — not ending in a newline — so the collapsed branch
+      // of the reveal keeps a real engine under it; the case above owns the
+      // after-newline branch.
+    }, `\n${DRAFT}`)
     await expect.poll(async () => (await measureComposer(page)).overflows, { timeout: 10_000 }).toBe(true)
     await expect.poll(async () => (await measureComposer(page)).scrollTop, { timeout: 10_000 }).toBeGreaterThan(0)
     const pasted = await measureComposer(page)