chore(webui): remove /preview/* routes + rename nav_consolidated → nav_main

[nextlevelshit]

What

Epic #1624 WebUI UX audit constraint: /preview/* routes must be removed once consolidated (preview becomes prod). Also naming hygiene.

Changes

1. Delete internal/webui/templates/preview/ — 6 files. Routes already removed from routes.go.
2. Delete internal/webui/preview.go — main preview package (not stub).
3. Delete internal/webui/preview_services.go
4. Delete internal/webui/preview_test.go
5. Keep internal/webui/preview_disabled.go — //go:build !webui_preview stub, no-op for non-preview builds.
6. Rename nav_consolidated.html → nav_main.html — proper noun not process descriptor.

Constraints

• API contract unchanged (no handler/data model changes)
• All work targets alpha branch
• Commit signed via CLAUDE.md gitsign hook (wave pipeline, not worktree shortcut)

Acceptance

• go build ./internal/webui/ passes (no webui_preview tag)
• go build -tags=webui_preview ./internal/webui/ passes
• No templates/preview/ directory remains
• No preview.go, preview_services.go, preview_test.go remain
• nav_main.html exists, nav_consolidated.html gone