FN-5706: hide chat input overflow until max-height

Prevent the chat composer from showing a right-edge scrollbar line before it reaches the autosize cap.

- set chat textarea default overflow to hidden in ChatView CSS
- add resolveChatInputOverflowY to switch overflow to auto only past the 640px cap
- apply overflow mode during composer autosize updates
- extend chat input autosize tests and docs to cover overflow behavior

Files changed:
 docs/dashboard-guide.md                                          | 2 +-
 packages/dashboard/app/components/ChatView.css                   | 1 +
 packages/dashboard/app/components/ChatView.tsx                   | 5 +++++
 packages/dashboard/app/components/__tests__/ChatView.chat-input-autosize.test.tsx   | 9 ++++++++-
 4 files changed, 15 insertions(+), 2 deletions(-)

Fusion-Task-Id: FN-5706

Fusion-Task-Lineage: 4619aa30-c02f-453b-bdf2-f60f360e9372

docs/dashboard-guide.md

@@ -1078,7 +1078,7 @@ const baseOnly = await loadAllAppCssBaseOnly(); // strips @media/@supports
 ### File browser editor & autosize textarea
 
 - `FileEditor.tsx` is CodeMirror 6-only (no `<textarea>` fallback). Language resolution: `packages/dashboard/app/utils/codemirror-language.ts`.
-- For chat-style composer fields use `packages/dashboard/app/hooks/useAutosizeTextarea.ts`. Pattern: `height = "auto"` then clamp `scrollHeight` to min/max in `useLayoutEffect`. Pair with `resize: none` and `overflow-y: auto`.
+- For chat-style composer fields use `packages/dashboard/app/hooks/useAutosizeTextarea.ts`. Pattern: `height = "auto"` then clamp `scrollHeight` to min/max in `useLayoutEffect`. Pair with `resize: none`; keep `overflow-y: hidden` while under the max-height cap and switch to `overflow-y: auto` only after content exceeds the cap.
 
 ### File-path links