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.

backend/src/text_gen.py

@@ -127,19 +127,18 @@ 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 started writing a prompt and wants you to turn it into a complete, polished prompt. Here is what they have so far:
+The user has written the beginning of a prompt and wants you to continue it. Here is what they have:
 
 "{user_input}"
 
-Rewrite and expand their idea into a single detailed, evocative music generation prompt. Your result should:
-- Incorporate the user's existing words naturally — they should recognize their idea in the result
-- Expand with specific details about genre, instrumentation, mood, tempo, structure, or production style
-- Read as one complete, cohesive paragraph ready to use as-is
-- Be between 2-4 sentences total
+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:
 
-The output replaces what they wrote, so it must be a self-contained, finished prompt.
+- 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
 
-Output ONLY the completed prompt text. No numbers, no labels, no commentary."""
+Output ONLY your continuation 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).