feat: add per-app usage filtering (Claude/Codex/Gemini)

Add dashboard-level app type filter to usage statistics, replacing the
DataSourceBar with a more useful segmented control. All components
(summary cards, trend chart, provider stats, model stats, request logs)
now respond to the selected app filter.

Backend: add optional app_type parameter to get_usage_summary,
get_daily_trends, get_provider_stats, and get_model_stats queries.
Frontend: new AppTypeFilter type, updated query keys with appType
dimension for proper cache separation, and RequestLogTable local
filter auto-locks when dashboard filter is active.

src/i18n/locales/en.json

@@ -1019,6 +1019,14 @@
     "stream": "Stream",
     "nonStream": "Non-stream",
     "source": "Source",
+    "requestsLabel": "requests",
+    "costLabel": "total cost",
+    "appFilter": {
+      "all": "All",
+      "claude": "Claude Code",
+      "codex": "Codex",
+      "gemini": "Gemini"
+    },
     "dataSources": "Data Sources",
     "dataSource": {
       "proxy": "Proxy",

src/i18n/locales/ja.json

@@ -1019,6 +1019,14 @@
     "stream": "ストリーム",
     "nonStream": "非ストリーム",
     "source": "ソース",
+    "requestsLabel": "リクエスト",
+    "costLabel": "合計コスト",
+    "appFilter": {
+      "all": "すべて",
+      "claude": "Claude Code",
+      "codex": "Codex",
+      "gemini": "Gemini"
+    },
     "dataSources": "データソース",
     "dataSource": {
       "proxy": "プロキシ",

src/i18n/locales/zh.json

@@ -1019,6 +1019,14 @@
     "stream": "流",
     "nonStream": "非流",
     "source": "来源",
+    "requestsLabel": "次请求",
+    "costLabel": "总成本",
+    "appFilter": {
+      "all": "全部",
+      "claude": "Claude Code",
+      "codex": "Codex",
+      "gemini": "Gemini"
+    },
     "dataSources": "数据来源",
     "dataSource": {
       "proxy": "代理",