feat: Plan-Modus – Toggle im Chat zum Umschalten zwischen Plan/Ausführung

- Toggle-Button (🗺️) im Chat-Input-Bereich neben Explorer
- Plan-Modus wird per ChatRequest.planMode an Backend gesendet
- Backend: Write-Tools gefiltert (nur read_file, list_files, glob, grep,
  todo_write, memory_*, ask_user_choice bleiben), System-Prompt-Block
  ergänzt der Änderungen verbietet, tool_choice nie 'any' erzwungen
- ChainRecord.PlanMode: pro Chain gespeichert (informativ für History)
- Chain-Meta-Badge '📋 Plan-Modus' bei Plan-Chains
- Choice-Response erbt planMode vom vorherigen Chain
- CSS: .plan-mode-active Styling (Primary-Color Highlight)
- Placeholder wechselt zu 'Plannen...' wenn aktiv
- Plan-Modus wird bei Session-Wechsel zurückgesetzt

Services/DatabaseService.cs

@@ -179,6 +179,7 @@ public class DatabaseService
         TryAddColumn(conn, "agents", "avatar_offset_y", "REAL NOT NULL DEFAULT 0");
         TryAddColumn(conn, "sessions", "finalizer_agent_id", "TEXT");
         TryAddColumn(conn, "agents", "allow_user_choice", "INTEGER NOT NULL DEFAULT 1");
+        TryAddColumn(conn, "chains", "plan_mode", "INTEGER NOT NULL DEFAULT 0");
 
         conn.Execute(@"
             CREATE TABLE IF NOT EXISTS agent_skills (
@@ -579,8 +580,8 @@ public class DatabaseService
         using var conn = CreateConnection();
         chain.CreatedAt = DateTime.UtcNow.ToString("o");
         await conn.ExecuteAsync(@"
-            INSERT INTO chains (chain_id, session_id, archive_id, agent_id, model, system_prompt, tool_choice, tool_names, input_tokens, output_tokens, iterations, stop_reason, duration_ms, created_at)
-            VALUES (@ChainId, @SessionId, @ArchiveId, @AgentId, @Model, @SystemPrompt, @ToolChoice, @ToolNames, @InputTokens, @OutputTokens, @Iterations, @StopReason, @DurationMs, @CreatedAt)
+            INSERT INTO chains (chain_id, session_id, archive_id, agent_id, model, system_prompt, tool_choice, tool_names, input_tokens, output_tokens, iterations, stop_reason, duration_ms, plan_mode, created_at)
+            VALUES (@ChainId, @SessionId, @ArchiveId, @AgentId, @Model, @SystemPrompt, @ToolChoice, @ToolNames, @InputTokens, @OutputTokens, @Iterations, @StopReason, @DurationMs, @PlanMode, @CreatedAt)
         ", chain);
     }