Dependency graph visualization for monorepos (like nx graph)

## Feature Request

### Problem

While migrating [TresJS](https://github.com/Tresjs/tres) (a monorepo with 15 workspace packages) to Vite+, running `vp run build -r` only picked up 9 of the 15 packages. There was no way to check which packages Vite+ actually sees in its dependency graph, making it hard to tell whether the missing packages were misconfigured, not recognized as part of the workspace, or just not connected in the dependency tree.

Without a way to visualize what Vite+ understands about the monorepo structure, debugging this kind of issue becomes trial and error.

### Proposed Solution

Add a `vp graph` command (similar to `nx graph`) that shows the monorepo's dependency graph as Vite+ sees it.

**Core features:**
1. **Interactive web UI** — opens a browser with a visual DAG of all workspace packages and their internal dependencies
2. **Filter by package** — focus on a specific package to see its dependents/dependencies
3. **Affected view** — highlight which packages are affected by changes
4. **Export options** — JSON output for CI/tooling integration (`vp graph --json`)

### Example (Nx reference)

`nx graph` opens an interactive browser UI showing:
- All projects as nodes
- Dependencies as directed edges
- Filtering, searching, and grouping capabilities
- "Affected" overlay showing what changed

### Why

- Debugging missing/unrecognized packages in large monorepos is painful without this
- Complements the existing topological task scheduling
- Helps catch circular dependency issues early
- Related to #1167 (consolidated output) and #1098 (monorepo DX improvements)

### Validations

- [x] Read the [Contributing Guidelines](https://github.com/voidzero-dev/vite-plus/blob/main/CONTRIBUTING.md).
- [x] Confirm this request is for Vite+ itself and not for Vite, Vitest, tsdown, Rolldown, or Oxc.
- [x] Check that there isn't already an issue requesting the same feature.