release: prepare Desktop 0.15.6

Include the pending sidebar footer style correction and document frosted appearance, chat rendering, image previews, settings layout, and local MCP recovery. Record schema 52 backup guidance and 886 passing focused tests; Agent remains 0.15.1.

Release note: 新增可选磨砂外观，改善聊天输入、图片预览和本地工具连接恢复，并整理设置与笔记布局。

FEATURES.md

@@ -333,7 +333,7 @@ records are listed separately and do not introduce another feature status.
   failure, Agent `SIGKILL`/restart, and recovery from a reopened Desktop SQLite
   database. Successful tool START/END events appear exactly once, with no
   Prompt, provider, or tool replay observed. The current Agent source lock is
-  `0.15.1`, paired with the current Desktop release candidate `0.15.5`; formal
+  `0.15.1`, paired with the current Desktop release candidate `0.15.6`; formal
   publication status follows the separate Agent and Desktop
   release channels. Previous macOS validation covered native package installation,
   detached lifecycle, Attach, real Ask/Execute, and cancellation of tools in
@@ -951,9 +951,18 @@ records are listed separately and do not introduce another feature status.
 
 ### Open source, builds, and releases
 
-- The current Desktop candidate is `0.15.5`; published Agent `0.15.1` is unchanged, with
+- The current Desktop candidate is `0.15.6`; published Agent `0.15.1` is unchanged, with
   OpenCode pinned to `1.18.29` and Continue to `1.5.47`. Publication status follows the independent
   Desktop and Agent release channels.
+- Desktop `0.15.6` adds optional, persisted frosted-glass header/checklist surfaces
+  (off by default, without OS desktop transparency), reduces repeated chat renders,
+  and supports direct previews for draft/queued images. Settings are grouped with
+  right-aligned switches, and note headers/editor spacing are tighter.
+  Local OpenCode retries explicit transient MCP transport failures once after
+  cleanup and retires uncertain names; it does not replay prompts or tool calls.
+  Redacted MCP diagnostics support feedback. Schema 52 adds execution-statistics
+  indexes; back up the database and notes before upgrading.
+  See the [preparation record](./docs/development/release-preparation-0.15.6.md).
 - Desktop `0.15.5` updates Smart heartbeat and incremental review, preserves
   cancelled-turn history, floats checklist details, and exits note editing after
   saving. It also adds manual note tags with AND filtering and rename/merge/delete,