feat(init): add --user flag for non-admin Windows installation

[CalvinAllen]

Summary

• Add --user flag to dtvem init for non-admin Windows installation
• Uses User PATH instead of System PATH (no admin required)
• Warns when system runtimes would shadow dtvem shims

Changes

File	Description
src/internal/config/settings.go	Settings struct and persistence
src/internal/config/settings_test.go	Unit tests for settings
src/cmd/init.go	--user flag, settings persistence, switch detection
src/cmd/init_test.go	Unit tests for init command
src/internal/path/path_windows.go	User PATH support, conflict detection
src/internal/path/path_unix.go	Signature update for API consistency
install.ps1	-UserInstall parameter
install.sh	--user-install argument
schemas/settings.schema.json	JSON Schema for settings file
.github/workflows/integration-test-init.yml	E2E tests for init command

Usage

# Windows - Standard (admin required)
irm dtvem.io/install.ps1 | iex

# Windows - User install (no admin)
iex "& { $(irm dtvem.io/install.ps1) } -UserInstall"

# Direct command
dtvem init --user      # User PATH
dtvem init             # System PATH (admin)

Test Plan

• Unit tests pass (go test ./...)
• dtvem init --user -y creates settings.json with "installType": "user"
• Settings persist across init calls
• Conflict detection warns about system runtimes
• E2E tests (requires feat(workflows): add dtvem-integration-test-init.yml .github#52 merged first)

Closes #204