fix(agent): correct media token arithmetic and tool call double-counting

Two estimation bugs fixed:

1. Media tokens were added to the chars accumulator before the chars*2/5
   conversion, resulting in 256*2/5=102 tokens per item instead of 256.
   Fix: add media tokens directly to the final token count, bypassing
   the character-based heuristic.

2. estimateMessageTokens counted both tc.Name and tc.Function.Name for
   tool calls, but providers only send one (OpenAI-compat uses
   function.name, Anthropic uses tc.Name). Fix: count tc.Function.Name
   when Function is present, fall back to tc.Name only otherwise.

Also fix i18n hint text: "auto-detect" was misleading — the backend
uses a 4x max_tokens heuristic, not actual model detection.

web/frontend/src/i18n/locales/en.json

@@ -397,7 +397,7 @@
       "max_tokens": "Max Tokens",
       "max_tokens_hint": "Upper token limit per model response.",
       "context_window": "Context Window",
-      "context_window_hint": "Model input context capacity in tokens. Leave empty to auto-detect (default: 4x max tokens).",
+      "context_window_hint": "Model input context capacity in tokens. Leave empty to use the default (4x max tokens).",
       "max_tool_iterations": "Max Tool Iterations",
       "max_tool_iterations_hint": "Maximum tool-call loops in a single task.",
       "summarize_threshold": "Summarize Message Threshold",

web/frontend/src/i18n/locales/zh.json

@@ -397,7 +397,7 @@
       "max_tokens": "最大 Token 数",
       "max_tokens_hint": "单次模型响应允许的最大 Token 数。",
       "context_window": "上下文窗口",
-      "context_window_hint": "模型输入上下文容量（Token 数）。留空则自动推算（默认为最大 Token 数的 4 倍）。",
+      "context_window_hint": "模型输入上下文容量（Token 数）。留空使用默认值（最大 Token 数的 4 倍）。",
       "max_tool_iterations": "最大工具迭代次数",
       "max_tool_iterations_hint": "单个任务中允许的工具调用循环上限。",
       "summarize_threshold": "触发摘要的消息阈值",