fix: Keep Composing appends continuation instead of replacing prompt

Two fixes:
- Backend: MAESTRO_COMPLETE_PROMPT now instructs Gemini to output ONLY
  the continuation text, not a rewrite. No repeating user's words.
- Frontend: complete mode appends result to existing prompt with smart
  separator logic (lowercase start = space, uppercase = period+space).
  generate mode still replaces the empty prompt as before.

frontend/src/components/Generator.tsx

@@ -188,10 +188,21 @@ export default function Generator({ onGenerate }: Props) {
               <MaestroButton
                 prompt={prompt}
                 onResult={(result, mode) => {
-                  setPrompt(result)
-                  if (mode === 'generate') {
-                    setError('')
+                  if (mode === 'complete') {
+                    // Append continuation to existing prompt
+                    setPrompt(prev => {
+                      const trimmed = prev.trimEnd()
+                      // If result starts lowercase, it flows from the input — just space
+                      // If result starts uppercase, treat as new sentence — period + space
+                      const startsLower = /^[a-z]/.test(result)
+                      const separator = startsLower ? ' ' : '. '
+                      return trimmed + separator + result
+                    })
+                  } else {
+                    // Generate mode: replace empty prompt
+                    setPrompt(result)
                   }
+                  setError('')
                 }}
                 onError={(msg) => setError(msg)}
               />
@@ -214,9 +225,19 @@ export default function Generator({ onGenerate }: Props) {
                 prompt={Object.values(structuredPrompt).filter(Boolean).join(' ')}
                 structured={isPro}
                 onResult={(result, mode) => {
-                  // Fallback: plain text mode for non-Pro, put result in 'additional'
-                  setStructuredPrompt(prev => ({ ...prev, additional: result }))
-                  if (mode === 'generate') setError('')
+                  if (mode === 'complete') {
+                    // Fallback: append to additional field
+                    setStructuredPrompt(prev => {
+                      const existing = prev.additional?.trim() || ''
+                      const startsLower = /^[a-z]/.test(result)
+                      const separator = startsLower ? ' ' : '. '
+                      return { ...prev, additional: existing + separator + result }
+                    })
+                  } else {
+                    // Generate: put in additional
+                    setStructuredPrompt(prev => ({ ...prev, additional: result }))
+                  }
+                  setError('')
                 }}
                 onStructuredResult={(fields, mode) => {
                   // Structured mode: merge fields into structuredPrompt, preserving user edits