feat(installer): optional data cleanup checkbox in Windows uninstaller

Add a Yes/No MessageBox to the NSIS customUnInstall macro that offers
to remove ~/.gaia/ (chats, documents, Python venv, config, logs)
during uninstall. Default is No so users keep their data unless they
explicitly opt in. Silent uninstalls (/SD IDNO) also default to
keeping data.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kovtcharov

installer/nsis/installer.nsh

@@ -69,4 +69,22 @@
 !macro customUnInstall
   DeleteRegValue HKCU "Software\Microsoft\Windows\CurrentVersion\Run" \
       "${PRODUCT_NAME}"
+
+  ; Offer to remove user data (~/.gaia/) — chats, documents, Python venv,
+  ; config, and logs. Default is "No" so users keep their data unless they
+  ; explicitly opt in. This matches the tiered uninstall design in §5 of
+  ; the desktop installer plan.
+  MessageBox MB_YESNO|MB_ICONQUESTION \
+      "Also remove your GAIA data (chats, documents, Python environment)?$\r$\n$\r$\nThis cannot be undone." \
+      /SD IDNO \
+      IDYES _gaiaRemoveData IDNO _gaiaKeepData
+
+  _gaiaRemoveData:
+    RmDir /r "$PROFILE\.gaia"
+    Goto _gaiaDone
+
+  _gaiaKeepData:
+    ; User chose to keep data — nothing to do.
+
+  _gaiaDone:
 !macroend