feat: add Script Builder page with chat UI, code blocks, preview modal, and save dialog

- ScriptCodeBlock: collapsed code preview with syntax highlighting (first 5 lines)
- ScriptBuilderInput: auto-resize chat input with Enter-to-send
- ScriptBuilderChat: message list with markdown rendering and code blocks
- ScriptPreviewModal: fullscreen script viewer with line numbers
- SaveToLibraryDialog: save script with name, description, category, team sharing
- ScriptBuilderPage: language selector, session management, FlowPilot handoff
- Added route, sidebar nav item (fuchsia), and mobile nav entry

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/script-builder/ScriptBuilderInput.tsx

@@ -0,0 +1,78 @@
+import { useState, useRef, useCallback, useEffect } from 'react'
+import { Send } from 'lucide-react'
+import { cn } from '@/lib/utils'
+
+interface ScriptBuilderInputProps {
+  onSend: (content: string) => void
+  disabled: boolean
+  placeholder?: string
+}
+
+export function ScriptBuilderInput({
+  onSend,
+  disabled,
+  placeholder = 'Describe the script you need...',
+}: ScriptBuilderInputProps) {
+  const [value, setValue] = useState('')
+  const textareaRef = useRef<HTMLTextAreaElement>(null)
+
+  const adjustHeight = useCallback(() => {
+    const textarea = textareaRef.current
+    if (!textarea) return
+    textarea.style.height = 'auto'
+    textarea.style.height = `${Math.min(textarea.scrollHeight, 120)}px`
+  }, [])
+
+  useEffect(() => {
+    adjustHeight()
+  }, [value, adjustHeight])
+
+  const handleSend = () => {
+    const trimmed = value.trim()
+    if (!trimmed || disabled) return
+    onSend(trimmed)
+    setValue('')
+  }
+
+  const handleKeyDown = (e: React.KeyboardEvent<HTMLTextAreaElement>) => {
+    if (e.key === 'Enter' && !e.shiftKey) {
+      e.preventDefault()
+      handleSend()
+    }
+  }
+
+  const canSend = value.trim().length > 0 && !disabled
+
+  return (
+    <div className="flex items-end gap-2 p-3 border-t" style={{ borderColor: 'var(--glass-border)' }}>
+      <textarea
+        ref={textareaRef}
+        value={value}
+        onChange={(e) => setValue(e.target.value)}
+        onKeyDown={handleKeyDown}
+        placeholder={placeholder}
+        disabled={disabled}
+        rows={1}
+        className={cn(
+          "flex-1 resize-none rounded-xl px-4 py-2.5 text-sm",
+          "bg-card border border-border text-foreground placeholder:text-muted-foreground",
+          "focus:outline-none focus:border-[rgba(6,182,212,0.3)] transition-colors",
+          "disabled:opacity-50"
+        )}
+        style={{ maxHeight: 120 }}
+      />
+      <button
+        onClick={handleSend}
+        disabled={!canSend}
+        className={cn(
+          "shrink-0 flex items-center justify-center w-10 h-10 rounded-xl transition-all",
+          canSend
+            ? "bg-gradient-brand text-[#101114] hover:opacity-90 active:scale-[0.97]"
+            : "bg-[rgba(255,255,255,0.04)] text-[#5a6170] cursor-not-allowed"
+        )}
+      >
+        <Send size={18} />
+      </button>
+    </div>
+  )
+}