fix: move punctuation logic to LLM - backend returns complete joined text

The frontend no longer does separator/punctuation logic. Both Maestro
modes (generate and complete) now simply setPrompt(result). The LLM
is instructed to return the user's exact words + continuation, properly
joined. This is the right layer for text joining decisions.

backend/src/text_gen.py

@@ -127,18 +127,15 @@ Output ONLY the prompt text. No numbers, no labels, no commentary."""
 
 MAESTRO_COMPLETE_PROMPT = """You are Maestro, an expert music prompt writer for AI music generation models (specifically Google Lyria).
 
-The user has written the beginning of a prompt and wants you to continue it. Here is what they have:
+The user has started writing a prompt and wants you to continue it. Here is what they have:
 
 "{user_input}"
 
-Continue and expand their idea. Add 2-3 sentences of specific musical detail — genre, instrumentation, mood, tempo, structure, production style. Your continuation will be appended directly after their text, so:
+Continue their idea by adding 2-3 sentences of specific musical detail — genre, instrumentation, mood, tempo, structure, production style.
 
-- Do NOT repeat or rephrase what they wrote
-- Do NOT add a period if their text ends mid-sentence
-- Start naturally as if picking up from where they left off
-- Be vivid and specific — a producer should be able to generate from this
+Return the COMPLETE prompt: the user's EXACT words, followed by your continuation, properly joined. Do not rephrase, rewrite, or change any of the user's words. Just continue from where they left off.
 
-Output ONLY your continuation text. No labels, no commentary, no quotes."""
+Output ONLY the complete prompt text. No labels, no commentary, no quotes."""
 
 
 MAESTRO_STRUCTURED_PROMPT = """You are Maestro, an expert music prompt writer for AI music generation models (specifically Google Lyria).

frontend/src/components/Generator.tsx

@@ -187,23 +187,8 @@ export default function Generator({ onGenerate }: Props) {
               <label className="text-xs text-kithara-muted">Prompt</label>
               <MaestroButton
                 prompt={prompt}
-                onResult={(result, mode) => {
-                  if (mode === 'complete') {
-                    // Append continuation to existing prompt
-                    setPrompt(prev => {
-                      const trimmed = prev.trimEnd()
-                      const endsWithPunct = /[.!?,;:]$/.test(trimmed)
-                      const startsLower = /^[a-z]/.test(result)
-                      // Already punctuated? Just space.
-                      // Flows naturally (lowercase)? Just space.
-                      // Otherwise: period + space for a new sentence.
-                      const separator = (endsWithPunct || startsLower) ? ' ' : '. '
-                      return trimmed + separator + result
-                    })
-                  } else {
-                    // Generate mode: replace empty prompt
-                    setPrompt(result)
-                  }
+                onResult={(_result, _mode) => {
+                  setPrompt(_result)
                   setError('')
                 }}
                 onError={(msg) => setError(msg)}
@@ -226,22 +211,7 @@ export default function Generator({ onGenerate }: Props) {
               <MaestroButton
                 prompt={Object.values(structuredPrompt).filter(Boolean).join(' ')}
                 structured={isPro}
-                onResult={(result, mode) => {
-                  if (mode === 'complete') {
-                    // Fallback: append to additional field
-                    setStructuredPrompt(prev => {
-                      const existing = prev.additional?.trim() || ''
-                      const endsWithPunct = /[.!?,;:]$/.test(existing)
-                      const startsLower = /^[a-z]/.test(result)
-                      const separator = (endsWithPunct || startsLower) ? ' ' : '. '
-                      return { ...prev, additional: existing + separator + result }
-                    })
-                  } else {
-                    // Generate: put in additional
-                    setStructuredPrompt(prev => ({ ...prev, additional: result }))
-                  }
-                  setError('')
-                }}
+                onResult={(_result, _mode) => { /* handled by onStructuredResult */ }}
                 onStructuredResult={(fields, mode) => {
                   // Structured mode: merge fields into structuredPrompt, preserving user edits
                   setStructuredPrompt(prev => ({