CrossRelay stage 0+1: Go gateway with Anthropic<->Chat Completions relay, Tauri shell, bilingual management UI

- Go gateway (stdlib only): /v1/messages non-stream + SSE, /v1/messages/count_tokens,
  management API /api/* (providers CRUD, apply/restore, settings, backups, logs)
- Protocol conversion: Anthropic Messages <-> Chat Completions with a streaming
  state machine keyed by content block index (text, tool_use, parallel calls)
- ~/.claude/settings.json guarded read/write: timestamped backups, managed env
  keys only, one-click restore
- Tauri 2 shell: spawns the gateway sidecar, parses the random port from stdout,
  stdin pipe watchdog prevents orphans, kills the child on exit
- React frontend: Dashboard/Providers/Settings, zh-CN + en-US, TanStack Query
- Tests: Go table-driven + httptest integration (all green), vitest + RTL

src-tauri/capabilities/default.json

@@ -0,0 +1,17 @@
+{
+  "$schema": "../gen/schemas/desktop-schema.json",
+  "identifier": "default",
+  "description": "Default capabilities for CrossRelay: local gateway HTTP access.",
+  "windows": ["main"],
+  "permissions": [
+    "core:default",
+    "core:event:default",
+    {
+      "identifier": "http:default",
+      "allow": [
+        { "url": "http://127.0.0.1:*" },
+        { "url": "http://localhost:*" }
+      ]
+    }
+  ]
+}