feat: support complete DeepSeek Harness provider management in CLI and TUI #501

Feature Request

Support DeepSeek Harness (DSH) in CC-Switch CLI and TUI using the existing provider workflows for Claude Code, Codex, and Gemini.

Expected Behavior

• Select --app dsh, with deepseek and deepseek-harness aliases, app visibility settings, and a distinct accent.
• Add, edit, copy, list, inspect, import, switch, set a default model, and delete saved providers.
• Manage initialized profile, API key, Base URL, protocol, model catalog, default model, reasoning effort, and native advanced options. Support DeepSeek and the OpenAI Completions/Responses and Anthropic Messages adapters.
• Discover models, check endpoints, query custom usage scripts, and export private JSON for reimport.
• Preserve native plugins, dynamic expressions, comments, other routes, and browser grants. Publish patches atomically, keep credentials in the native store, cooperate with native locks, and roll back failed switches.
• Validate generated configuration against native DSH semantics and cover CLI/TUI, failure, and concurrency workflows with isolated tests and English/Chinese documentation.

Proxy takeover, failover, MCP, prompts, skills, sessions, and common snippets are outside this provider-management request. Shared cross-app persistence redesign is also outside its scope.

Related Implementation

PR #500 implements this request and records validation, baseline test/Clippy failures, and runtime limitations.