feat(ai): upgrade agent loop tool calling to true SSE streaming

Move tool calling from non-streaming (complete) API calls to real
streaming SSE, so text, reasoning, and tool call arguments all arrive
incrementally — users see the AI think and act in real time.

- Extract ~370 lines of non-streaming tool-call functions from
  agent_loop.rs (call_with_tools, call_openai_with_tools,
  call_claude_with_tools, call_gemini_with_tools) and replace with
  a single ai::stream_with_tools() dispatch call.

- Add streaming-with-tools infrastructure to ai.rs (~550 lines):
  - StreamToolEvent enum: Chunk, ToolCallStart, ToolCallDelta,
    ToolCallComplete — provider-agnostic, feeds into the
    StreamingToolCallAccumulator
  - stream_claude_with_tools(): parse SSE content_block_start/delta/
    stop events, incremental input_json_delta for tool arguments
  - stream_openai_with_tools(): parse SSE delta.tool_calls with
    incremental function.arguments
  - stream_gemini_with_tools(): parse SSE functionCall (Gemini
    sends complete objects, not deltas — emit as one chunk)
  - Cancellation support via tokio::select! in every streaming loop
  - Extended timeout for reasoning models (600s vs 120s)

- StreamingToolCallAccumulator: collect partial tool call fragments
  from streaming deltas, then deserialize into complete ToolCall
  objects in index order once the stream ends.

apps/desktop/src/components/editor/AiAssistant.vue

@@ -253,8 +253,10 @@ function agentStepClass(tone: AiAgentStepTone): string {
 }
 
 function agentStepTitle(step: AiAgentStepItem): string {
-  if (!step.titleKey) return t(step.labelKey);
-  return t(step.titleKey, step.titleParams || {});
+  if (step.titleKey) return t(step.titleKey, step.titleParams || {});
+  const tool = step.titleParams?.tool;
+  if (tool) return `${t(step.labelKey)}: ${tool}`;
+  return t(step.labelKey);
 }
 
 function toggleReasoning(index: number) {
@@ -513,6 +515,22 @@ async function send() {
         if (event.type === "reasoning_delta" && event.delta) {
           appendAssistantReasoning(assistantIdx, event.delta);
         }
+        // Real-time agent step rendering
+        if (event.type === "tool_call_start" || event.type === "tool_call_end") {
+          const msg = messages.value[assistantIdx];
+          if (msg) {
+            const steps = agentEvents
+              .filter((e) => e.type === "tool_call_start" || e.type === "tool_call_end")
+              .map((e) => ({
+                key: `${e.tool_call_id || ""}-${e.type}`,
+                labelKey: e.type === "tool_call_start" ? "ai.agentSteps.callingTool" : e.is_error ? "ai.agentSteps.toolError" : "ai.agentSteps.toolDone",
+                tone: (e.type === "tool_call_start" ? "active" : e.is_error ? "danger" : "success") as AiAgentStepTone,
+                titleKey: undefined,
+                titleParams: { tool: e.tool_name || "" },
+              }));
+            msg.agentSteps = steps;
+          }
+        }
         scrollToBottom();
       },
       sessionId,