✨ Diagnostic Manager

### Description

Introduce a `DiagnosticManager` responsible for managing and publishing diagnostics (e.g., build errors, linter warnings, language server issues) grouped by file. This manager will serve as the single source of truth for diagnostics across the editor, and its contents will drive visual components such as the Issue Navigator and Line Annotations.

### Requirements

- Create a `DiagnosticManager` class conforming to `ObservableObject`
- Store diagnostics as a flat list or grouped by file:
```swift
@Published var issues: [DiagnosticIssue]
```
- Define a `DiagnosticIssue` model:
```swift
struct DiagnosticIssue: Identifiable, Hashable {
let id: UUID = UUID()
let file: URL
let line: Int
let column: Int?
let message: String
let severity: Severity
let source: String?
let fixItSuggestion: String?

enum Severity {
case error, warning, info, live
}
}
```
- Add convenience accessors:
```swift
var issuesByFile: [URL: [DiagnosticIssue]]
func issues(for file: URL) -> [DiagnosticIssue]
```
- Add a way to update issues by file, e.g.:
```swift
func setIssues(_ issues: [DiagnosticIssue], for file: URL)
func clearIssues(for file: URL)
```

### Additional Context

This manager will be used by multiple parts of the app, including the Issue Navigator and Line Annotations. It should be reactive and efficient, allowing UI components to automatically update when diagnostics change.

Future extensions may include filtering, debouncing updates, or supporting different diagnostics sources (build system, LSP, etc.).

### Related Issues

- #354
- #2015
- https://github.com/CodeEditApp/CodeEditSourceEditor/issues/297