feat(attachments): drag-and-drop file attachments on the detail pane

Drop a file anywhere on the detail pane to attach it: pane-wide drop target
with a 'Drop to attach' hover overlay (Copy cursor, gated on an idle selected
task), an explicit lingering confirmation/error line, plus an Attachments list
with size, remove, and an Add file… picker in the DETAILS card. ComposedPreview
now shows the reference files too. en/de keys added.

src/ClaudeDo.Localization/locales/en.json

@@ -182,7 +182,17 @@
     "descriptionPlaceholder": "Add task details (markdown supported)...",
     "prepTitle": "Daily prep",
     "planDay": "Plan day",
-    "prepEmpty": "No prep run today yet — click Plan day"
+    "prepEmpty": "No prep run today yet — click Plan day",
+    "attachments": {
+      "sectionLabel": "ATTACHMENTS",
+      "dropToAttach": "Drop to attach",
+      "addFile": "Add file…",
+      "removeTip": "Remove attachment",
+      "addedSummary": "✓ Added {0} ({1} file(s))",
+      "overLimitError": "Could not add {0}: {1}",
+      "invalidNameError": "Could not add {0}: {1}",
+      "selectIdleTask": "Select an idle task first"
+    }
   },
   "agent": {
     "stopTip": "Stop agent",