macOS: expand tilde in file paths before opening (#10863)

## Summary

Cmd-clicking a file path containing `~` (e.g. `~/Documents/file.txt`)
fails to open the file on macOS because `URL(filePath:)` treats `~` as a
literal directory name rather than the user's home directory.

This uses `NSString.expandingTildeInPath` to resolve `~` before
constructing the file URL.

## Root Cause

In `openURL()`, when the URL string has no scheme it falls through to:

```swift
url = URL(filePath: action.url)
```

Swift's `URL(filePath:)` does not perform tilde expansion. A path like
`~/Documents/file.txt` produces a URL pointing to a non-existent file,
and `NSWorkspace.open` silently fails.

## Fix

```swift
let expandedPath = NSString(string: action.url).expandingTildeInPath
url = URL(filePath: expandedPath)
```

## Reproduction

1. Have a terminal application (e.g. Claude Code) that outputs file
paths with `~` prefixes
2. Cmd-click the path in Ghostty on macOS
3. The file does not open (fails silently)

With this fix, the path resolves correctly and opens in the default
editor.

macos/Sources/Ghostty/Ghostty.App.swift

@@ -694,7 +694,10 @@ extension Ghostty {
             if let candidate = URL(string: action.url), candidate.scheme != nil {
                 url = candidate
             } else {
-                url = URL(filePath: action.url)
+                // Expand ~ to the user's home directory so that file paths
+                // like ~/Documents/file.txt resolve correctly.
+                let expandedPath = NSString(string: action.url).standardizingPath
+                url = URL(filePath: expandedPath)
             }
 
             switch action.kind {