feat(ui): Empty States für Detail-Pane und Task-Liste (UX-Audit #4)

Zentrierter, gedimmter Platzhalter (Mission-Control-Muster) im
Detail-Pane bei fehlender Selektion (nicht in Notes-/Prep-Modus) und
in der Task-Liste bei 0 sichtbaren Tasks; User-Listen ohne
WorkingDir bekommen zusätzlich den Repo-Verknüpfungs-Hinweis.

src/ClaudeDo.Localization/locales/en.json

@@ -189,7 +189,9 @@
     "feedbackPlaceholder": "What should the agent fix?",
     "rerun": "Re-run",
     "refineTip": "Refine this task with Claude",
-    "refiningTip": "Claude is refining this task…"
+    "refiningTip": "Claude is refining this task…",
+    "emptyHint": "No tasks yet. Add one above, or send it to the queue once you're ready.",
+    "emptyHintNoWorkingDir": "Link a repo in List Settings to enable the queue, Let Claude handle it, and Quick session."
   },
   "lists": {
     "heading": "Lists",
@@ -254,7 +256,8 @@
       "description": "Description",
       "steps": "Steps",
       "files": "Files"
-    }
+    },
+    "emptyState": "Select a task to see its details"
   },
   "agent": {
     "openWorktreeTip": "Open worktree in file explorer — requires an active worktree",