4. feat: remote proxy handler for desktop app

[Adam-D-Lewis]

Summary

• Add RemoteHandler that proxies frontend requests to a remote Nebi team server in local/desktop mode
• Uses existing cliclient package (typed methods) for all upstream communication
• Stores server URL and credentials in the local SQLite DB via store.Config/store.Credentials
• Routes registered only when localMode == true under /api/v1/remote/*

Endpoints (13 total)

• POST /connect — authenticate with remote server, store token
• GET /server — connection status
• DELETE /server — disconnect, clear credentials
• GET/POST/DELETE /workspaces[/:id] — proxy CRUD
• GET /workspaces/:id/versions — proxy version listing
• GET /workspaces/:id/tags — proxy tag listing
• GET /workspaces/:id/pixi-toml — proxy pixi.toml (JSON wrapped)
• GET /workspaces/:id/versions/:version/pixi-toml — proxy (JSON wrapped)
• GET /workspaces/:id/versions/:version/pixi-lock — proxy (JSON wrapped)
• POST /workspaces/:id/push — proxy version push

Design decisions

• 503 (not 400) for "not connected" errors — distinguishes client errors from config state issues
• JSON wrapping for pixi.toml/lock endpoints — upstream returns plain text, proxy wraps in {"content": "..."} for uniform frontend consumption
• GORM error checks on all DB writes in connect/disconnect
• Typed cliclient methods used where available (ListWorkspaces, GetWorkspace, etc.)

Test plan

• go vet ./...
• 6 unit tests: server status, connect/disconnect, mock remote, not-connected proxy
• go test ./internal/api/handlers/... ./internal/service/... ./internal/store/...
• Frontend builds cleanly