restructure codebase around LocalBackend with clear data ownership

[garmr-ulfr]

Summary

• Reorganize project architecture to establish clear data ownership and unidirectional dependency flow
• Introduce LocalBackend as central orchestrator, replacing the old Radiance struct
• Rename TunnelService to VPNClient with all VPN operations as methods
• Add CLI client (cmd/lantern) for interacting with the daemon over IPC

Package changes

• api/ → account/, replacing resty with net/http
• vpn/ipc/ → root-level ipc/ with separate client/server
• New backend/ package with LocalBackend
• New log/ package for logger creation and log streaming via PublishHandler
• Removed option/ package

Architecture

• Dependency flow is now ipc → vpn (not reverse)
• IPC server uses HTTP mux with traced handlers
• IPC client supports SSE streaming and mobile in-process fallback
• Settings use typed key constants; filename changed to settings.json

Bug fixes

• Fix event subscription collisions by keying on reflect.Type instead of zero-value instances
• Fix setData() change detection (&& → ||)
• Fix internal.LevelTrace → log.LevelTrace import errors in several packages

Other

• Simplify config.Config to alias ConfigResponse directly
• Flatten servers.Manager map to map[string]Server
• Remove go-resty/resty/v2, add alexflint/go-arg and golang.org/x/term

Test plan

• go build ./... passes
• go vet ./... passes
• go test ./... passes
• CLI client connects to daemon and executes subcommands
• Mobile in-process fallback works when IPC socket is unavailable