fix(chat): restore editor font size styling in chat input (#2199)

Commit 5c6546245 removed the editorFontSize reading and inline font-size
styling from ChatInput, calling it 'unused'. However this code was the
core of the editor font size feature (feat/e0229917f) — it applies the
user's chosen font size to the chat input textarea.

Without it, changing Settings > Appearance > Editor font size has no
effect on the chat input textarea. The CodeMirror editor (FilesView,
PlanView) was unaffected because its font size path was not touched.

This reverts the ChatInput portion of 5c6546245, restoring:
- useUIStore selector for editorFontSize
- inline fontSize style on the textarea element

Co-authored-by: bashrusakh <bashrusakh@users.noreply.github.com>

packages/ui/src/components/chat/ChatInput.tsx

@@ -1128,6 +1128,7 @@ const ChatInputComponent: React.FC<ChatInputProps> = ({ onOpenSettings, scrollTo
     const inputBarOffset = useUIStore((state) => state.inputBarOffset);
     const persistChatDraft = useUIStore((state) => state.persistChatDraft);
     const inputSpellcheckEnabled = useUIStore((state) => state.inputSpellcheckEnabled);
+    const editorFontSize = useUIStore((state) => state.editorFontSize);
     const isExpandedInput = useUIStore((state) => state.isExpandedInput);
     const setExpandedInput = useUIStore((state) => state.setExpandedInput);
     const setTimelineDialogOpen = useUIStore((state) => state.setTimelineDialogOpen);
@@ -5311,6 +5312,7 @@ const ChatInputComponent: React.FC<ChatInputProps> = ({ onOpenSettings, scrollTo
                                     maxHeight: !isComposerExpanded && textareaSize ? `${textareaSize.maxHeight}px` : undefined,
                                     borderTopLeftRadius: chatInputRadius,
                                     borderTopRightRadius: chatInputRadius,
+                                    fontSize: `${editorFontSize}px`,
                                 }}
                                 rows={1}
                             />