feat: Maestro Mode - context-aware prompt generation

- Backend: POST /api/text-gen/maestro endpoint with dual modes
  - Empty prompt → generate from scratch (temp 1.0)
  - Non-empty prompt → complete/extend (temp 0.85)
  - structured=true → returns JSON fields for Advanced/Pro mode
  - Three system prompts: MAESTRO_GENERATE, MAESTRO_COMPLETE, MAESTRO_STRUCTURED
  - Trailing sentence cleanup for plain text mode
  - JSON fallback parsing for structured mode

- Frontend: MaestroButton component
  - Context-aware labels: ♪ Maestro Mode (empty) / ♪ Keep Composing (has content)
  - Loading states: animating ♩ with Dreaming.../Composing...
  - structured prop for Pro mode → populates structured fields
  - onStructuredResult callback merges fields into StructuredPrompt

- UX: Replaced 'Get Suggestions ✨' with Maestro Mode
  - Suggestions required text in the box; Maestro works with or without
  - Single button, context-aware behavior, cleaner UI
  - Advanced/Pro mode gets structured fields from Maestro
  - Removed SuggestionsModal dependency

- Rename: 'Compose for Me' → 'Keep Composing'

frontend/src/components/MaestroButton.tsx

@@ -0,0 +1,98 @@
+import { useState } from 'react'
+import type { PromptStructure } from './StructuredPromptEditor'
+
+interface Props {
+  /** Current prompt text. Empty = generate mode, non-empty = complete mode. */
+  prompt: string
+  /** Called with the Maestro result — the generated or completed prompt. */
+  onResult: (result: string, mode: 'generate' | 'complete') => void
+  /** Called when Maestro encounters an error. */
+  onError: (message: string) => void
+  /** Optional CSS class for the button container. */
+  className?: string
+  /**
+   * If true, send structured=true and return structured fields.
+   * The onStructuredResult callback will be called instead of onResult.
+   */
+  structured?: boolean
+  /** Called with structured fields when structured mode returns. */
+  onStructuredResult?: (fields: PromptStructure, mode: 'generate' | 'complete') => void
+}
+
+export default function MaestroButton({
+  prompt,
+  onResult,
+  onError,
+  className = '',
+  structured = false,
+  onStructuredResult,
+}: Props) {
+  const [isMaestroing, setIsMaestroing] = useState(false)
+
+  const handleMaestro = async () => {
+    setIsMaestroing(true)
+    try {
+      const res = await fetch('/api/text-gen/maestro', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({
+          hint: prompt.trim(),
+          structured,
+        }),
+      })
+
+      if (!res.ok) {
+        try {
+          const errorData = await res.json()
+          throw new Error(errorData.detail || 'Maestro generation failed')
+        } catch (parseErr) {
+          if (parseErr instanceof Error && parseErr.message !== 'Maestro generation failed') throw parseErr
+          throw new Error('Maestro generation failed')
+        }
+      }
+
+      const data = await res.json()
+
+      if (structured && data.fields && onStructuredResult) {
+        onStructuredResult(data.fields as PromptStructure, data.mode)
+      } else {
+        onResult(data.prompt, data.mode)
+      }
+    } catch (err) {
+      onError(err instanceof Error ? err.message : 'Maestro generation failed')
+    } finally {
+      setIsMaestroing(false)
+    }
+  }
+
+  const mode = prompt.trim() ? 'complete' : 'generate'
+  const label = mode === 'generate'
+    ? '♪ Maestro Mode'
+    : '♪ Keep Composing'
+
+  return (
+    <button
+      type="button"
+      onClick={handleMaestro}
+      disabled={isMaestroing}
+      className={`${className} flex items-center justify-center gap-1.5 px-3 py-1.5 text-xs font-medium rounded-md transition-all duration-200 ${
+        isMaestroing
+          ? 'bg-kithara-accent/20 text-kithara-accent animate-pulse'
+          : 'bg-kithara-accent/10 text-kithara-accent hover:bg-kithara-accent/25 border border-kithara-accent/30 hover:border-kithara-accent/50'
+      }`}
+      title={mode === 'generate'
+        ? "Let Maestro write a creative prompt for you"
+        : "Let Maestro complete your prompt with musical details"
+      }
+    >
+      {isMaestroing ? (
+        <>
+          <span className="inline-block animate-spin">♩</span>
+          {mode === 'generate' ? 'Dreaming...' : 'Composing...'}
+        </>
+      ) : (
+        label
+      )}
+    </button>
+  )
+}