Upgrade default Claude Opus model to 4.8

Bump the default Opus route/model from claude-opus-4-7 to claude-opus-4-8
across provider presets (claude, claudeDesktop, hermes, openclaw, opencode,
universal), i18n locales (zh/en/ja/zh-TW), pricing seed data, and the
user-manual docs.

- Add claude-opus-4-8 pricing row ($5/$25/$0.50/$6.25); keep the 4-7 row
  for historical usage stats (seeded via INSERT OR IGNORE).
- Claude Desktop proxy: accept bidirectional opus 4-7 <-> 4-8 route alias
  during rollout so previously saved routes keep resolving.
- thinking_optimizer: route opus-4-8 through adaptive thinking and normalize
  dotted model ids (also fixes dotted 4-6/4-7 falling back to legacy).
- usage_stats: normalize Bedrock/Vertex/aggregator opus-4-8 ids to base
  pricing.

Also merge role:"system" messages into the Gemini systemInstruction in the
Anthropic->Gemini transform.

src-tauri/src/database/schema.rs

@@ -1205,6 +1205,15 @@ impl Database {
     /// 注意: model_id 使用短横线格式（如 claude-haiku-4-5），与 API 返回的模型名称标准化后一致
     fn seed_model_pricing(conn: &Connection) -> Result<(), AppError> {
         let pricing_data = [
+            // Claude 4.8 系列
+            (
+                "claude-opus-4-8",
+                "Claude Opus 4.8",
+                "5",
+                "25",
+                "0.50",
+                "6.25",
+            ),
             // Claude 4.7 系列
             (
                 "claude-opus-4-7",