feat: add message input with send button and keyboard shortcuts

- MessageInput component with textarea, send button, Enter-to-send
- Shift+Enter for newline, auto-resize textarea
- Disabled state while streaming, auto-focus after send
- Integrated into /chat page with user message handling

Closes #6

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

implementation-plans/_index.md

@@ -7,3 +7,4 @@
 | #3    | Configure Caddy for gRPC-Web support                   | COMPLETED    | [issue-003.md](issue-003.md) |
 | #4    | gRPC-Web client service layer                          | COMPLETED    | [issue-004.md](issue-004.md) |
 | #5    | Chat page layout and message list component            | COMPLETED    | [issue-005.md](issue-005.md) |
+| #6    | Message input with send and keyboard shortcuts         | COMPLETED    | [issue-006.md](issue-006.md) |

implementation-plans/issue-006.md

@@ -0,0 +1,17 @@
+---
+---
+
+# Issue #6: Message input with send and keyboard shortcuts
+
+**Status:** COMPLETED
+**Issue:** https://git.shahondin1624.de/llm-multiverse/llm-multiverse-ui/issues/6
+**Branch:** `feature/issue-6-message-input`
+
+## Acceptance Criteria
+
+- [x] Text input/textarea component for composing messages
+- [x] Send button triggers message submission
+- [x] Enter key sends message
+- [x] Shift+Enter inserts newline
+- [x] Input and send button disabled while a response is streaming
+- [x] Input auto-focuses on page load and after send