fix(ai): constrain chat input height to prevent button overflow

[lordspline]

This PR fixes an issue where typing long messages in the PostHog AI chat caused the input box to grow beyond the viewport, making the send button unreachable and the page unscrollable.

• Constrain textarea max-height using clamp(6rem, calc(100vh - 12rem), 16rem) to caps height at reasonable limits across all viewports
• Enable vertical scrolling within the textarea when content exceeds the cap
• Add test verifying the textarea has the constraint class applied

---

^{Need help on this PR? Tag /codesmith with what you need. Autofix is disabled.}

[capy-ai]

Visual QA — fix(ai): constrain chat input height

Tested against 07645fc8 (PR branch), compared to master. Built and ran locally at /project/1/ai (Max AI chat).

---

Empty state

No visual change — both before and after look identical ✅

---

Long message typed (desktop 1280×800)

Before (master)	Textarea grows to maxRows={10}; overflow happens silently with no scrollbar affordance
After (PR)	Textarea capped at max-height: 16rem (the top of the clamp range); a scrollbar appears, making overflow explicit

---

Long message typed (narrow 375×812)

Before (master)	Textarea grows tall, no scrollbar visible in the textarea itself
After (PR)	Clear vertical scrollbar visible; send button (→) remains accessible at the bottom-right corner

---

Key observations

• Empty state: visually identical ✅
• overflow-y: auto !important: scrollbar now appears on overflow — this is the most visible change ✅
• max-height: clamp(6rem, calc(100vh - 12rem), 16rem): at test viewports (800–812px height), the 16rem cap is the binding constraint. The 100vh - 12rem branch protects very short viewports (mobile keyboard open, ~300–450px) where maxRows={10} alone would push the send button off-screen ✅
• Send button (→): accessible in all tested states ✅
• No regressions on the suggestion chips or empty-state layout ✅

See screenshots in the next comment.

[capy-ai]

Narrow (375×812) — before vs after with long message:

Desktop (1280×800) — before vs after with long message: