Remove Splash syntax highlighter dependencyremove-splash-highlighter

Splash only highlights Swift, leaving every other language unhighlighted.
Drop the dependency entirely and render file contents as plain text until a
broader-coverage highlighter is adopted.

- Remove `import Splash` and the Splash-backed theming in FileTreeView
- Simplify CodeSyntaxHighlighter to emit plain attributed text
- Remove the Splash SwiftPM package reference and product dependency

Refs https://github.com/zerolabsco/hutch/issues/16

Co-Authored-By: Claude Opus 4.8 <[email protected]>

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