fix: prevent sync config from destroying externally-managed configs

[tylergraydev]

Summary

• Fixes [Bug]: Sync Config destroys .mcp.json #191 (reopened) — Sync Config was still overwriting .mcp.json, claude.json, opencode.json, and other editor configs with empty {} when a project had no MCPs in the app's database
• Imports MCPs from .mcp.json on project add — externally-configured servers now appear in the UI instead of showing an empty project
• Adds missing Tauri commands — open_folder and update_project_editor_type were invoked by the frontend but never implemented

Root cause

The previous fix (3064d5f) only guarded write_project_config() and write_global_config(), but missed write_project_to_claude_json() and all other editor config writers (OpenCode, Copilot, Cursor, Gemini, Codex). Additionally, .mcp.json was never read/imported when adding a project — the app only checked if the file existed but never parsed it.

Changes

File	Change
config_writer.rs	Guard write_project_to_claude_json against empty overwrite
opencode_config.rs	Guard both global and project writers
copilot_config.rs	Early return when mcps empty
cursor_config.rs	Early return when mcps empty
gemini_config.rs	Early return when mcps empty
codex_config.rs	Early return when mcps empty
scanner.rs	New import_mcps_from_project_mcp_json function
projects.rs	Import MCPs on add, fix .mcp.json path, add open_folder + update_project_editor_type commands
lib.rs	Register new commands

Test plan

• cargo check — 0 errors
• cargo test — 1991 tests pass (including updated preservation test)
• Manual: add a project with an existing .mcp.json → servers should appear in the UI
• Manual: click Sync Config on a project with no app-managed MCPs → .mcp.json should be preserved
• Manual: verify open_folder and editor type switching work in project dashboard

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