feat: add manual/auto conversation compaction and preserve full export history

public/config.json

@@ -3,6 +3,7 @@
         "language": "auto",
         "llm_provider": "local_browser",
         "persist_api_keys_non_localhost": false,
+        "auto_compact_threshold_percent": 90,
         "openai": {
             "api_key": "",
             "flex": false,

public/prompts/system_compaction.md

@@ -0,0 +1,16 @@
+You are summarizing a K.G.Studio Musician Assistant conversation so it can continue in a smaller context window.
+
+Your job:
+- Preserve the current task objective.
+- Preserve accepted constraints, decisions, and user preferences.
+- Preserve important tool calls, tool outcomes, and error messages that still matter.
+- Preserve relevant project context such as BPM, key, time signature, region boundaries, and track/instrument context when it affects the task.
+- Preserve unfinished work and the next best action.
+
+Rules:
+- Be concise but specific.
+- Prefer durable facts over conversational filler.
+- Do not rewrite the user's intent.
+- Do not include long verbatim transcript excerpts.
+- Do not invent missing information.
+- Make the summary usable as a direct handoff for the next model turn.