Add mvnlog --web: local web server for interactive build report viewing

## Context

PR #12699 (mvnlog: build log viewer, integration tests, script routing) introduced `mvnlog`, a CLI tool that renders build reports as formatted terminal output. While the terminal output is functional, a web-based viewer would provide a much richer experience for exploring build reports.

## Proposal

Add a `--web` flag to `mvnlog` that starts a lightweight local HTTP server serving an interactive build report viewer:

```bash
mvnlog --web # starts on localhost, auto-opens browser
mvnlog --web --port 8080 # custom port
```

## Implementation Plan

### Server (Java — zero external dependencies)

Use the JDK built-in `com.sun.net.httpserver.HttpServer` (available since Java 6, no additional dependencies):

- **`/`** — serves a single HTML page with inlined CSS + vanilla JS (bundled as a classpath resource)
- **`/api/report`** — returns the raw `build-report.json` content
- **`/api/reports`** — lists available build reports
- **Auto-open** — `java.awt.Desktop.browse(URI)` to open the browser
- **Self-terminating** — shuts down after 30 minutes of inactivity, or Ctrl+C
- **Port selection** — try default port, fall back to random available port if busy

### Frontend (single HTML file — no build step, no framework)

A single `report.html` resource bundled in the Maven distribution, containing:

#### Header
- Status badge (SUCCESS/FAILURE) with color
- Maven version, Java version, start time, total duration
- Goals executed, thread count

#### Module Timeline (Gantt chart)
- Horizontal bars showing parallel module builds
- Color-coded by status (green/red/gray)
- Hover for duration details
- Click to expand mojo breakdown

#### Mojo Detail Panel
- Click a module → see individual mojo executions
- Duration bars with proportional sizing
- Phase grouping (compile, test, package, etc.)
- Expandable log output per mojo

#### Problems Panel
- Filterable by severity (ERROR/WARN/INFO)
- Searchable by message text
- Each problem shows: key, source, suggestion (clickable), documentation URL (clickable link)
- Sortable by severity, source, key

#### Failures Panel
- Stack traces with syntax highlighting
- Module and mojo context
- Collapsible for long traces

#### Log Viewer
- Full build log with virtual scrolling (for large logs)
- Level-based filtering (ERROR/WARN/INFO/DEBUG)
- Module-scoped filtering
- Text search with highlighting

### Design Principles

- **No npm, no webpack, no React** — vanilla HTML/CSS/JS only, bundled as a single file
- **Responsive** — works on mobile (check build status from phone)
- **Dark/light mode** — respects `prefers-color-scheme`
- **Accessible** — proper ARIA roles, keyboard navigation
- **Offline** — everything inlined, no CDN dependencies
- **Small** — target < 50KB for the HTML resource

## Non-Goals (for this issue)

- Real-time streaming of ongoing builds (future enhancement)
- Multi-build comparison/history (future enhancement)
- Persistent server mode (future enhancement)
- Remote access / authentication (future enhancement)

## Depends on

- #12699 (mvnlog: build log viewer, integration tests, script routing)
- #12716 (mvnlog — needs serious rewrite)

Start by reviewing PR #12699 and the work tracked in #12716 to understand the existing mvnlog CLI and build-report format. Then scope the Java HttpServer integration and the bundled report.html resource around the proposed /, /api/report, and /api/reports endpoints. Done means --web supports port selection, browser opening, inactivity shutdown, and an offline interactive viewer covering the listed report panels without external dependencies.

Written by the indexing model from the issue text.