Add multi-language syntax highlighting (#16)

Splash (Swift-only, since removed) left code rendering unhighlighted
everywhere. Add Highlightr (highlight.js) behind a small SyntaxHighlighter
service and use it on both surfaces:

- Native file viewer: highlight NSAttributedString by language inferred
  from the filename, picking a light/dark theme from the trait style and
  re-highlighting on style changes. Falls back to plain text for unknown
  languages or very large files.
- Markdown/org code blocks in the WKWebView (READMEs, tickets, comments):
  the web view disables JavaScript, so highlight server-side into inline
  color-styled spans. The color scheme is now part of the render/cache
  key since colors are baked into the HTML.

Covered by unit tests for language mapping and highlighted output.

1 files changed, 48 insertions, 0 deletions

diff --git a/HutchTests/SyntaxHighlighterTests.swift b/HutchTests/SyntaxHighlighterTests.swift
new file mode 100644
index 0000000..258e81e
--- /dev/null
+++ b/HutchTests/SyntaxHighlighterTests.swift
@@ -0,0 +1,48 @@
+import Foundation
+import Testing
+import UIKit
+@testable import Hutch
+
+struct SyntaxHighlighterTests {
+    @Test
+    func mapsFileNamesToLanguages() {
+        #expect(SyntaxHighlighter.language(forFileName: "Sources/App/Main.swift") == "swift")
+        #expect(SyntaxHighlighter.language(forFileName: "server.py") == "python")
+        #expect(SyntaxHighlighter.language(forFileName: "index.tsx") == "typescript")
+        #expect(SyntaxHighlighter.language(forFileName: "Dockerfile") == "dockerfile")
+        #expect(SyntaxHighlighter.language(forFileName: "Makefile") == "makefile")
+    }
+
+    @Test
+    func returnsNilForUnknownExtensions() {
+        #expect(SyntaxHighlighter.language(forFileName: "notes.unknownext") == nil)
+        #expect(SyntaxHighlighter.language(forFileName: "LICENSE") == nil)
+    }
+
+    @Test
+    func highlightsKnownLanguageAsColoredSpans() {
+        let highlighter = SyntaxHighlighter(theme: .light)
+        let html = highlighter.highlightedHTML(for: "let answer = 42", language: "swift")
+
+        let unwrapped = try? #require(html)
+        #expect(unwrapped?.contains("<span style=\"color:") == true)
+        #expect(unwrapped?.contains("answer") == true)
+    }
+
+    @Test
+    func fallsBackToNilForUnsupportedLanguage() {
+        let highlighter = SyntaxHighlighter(theme: .dark)
+        #expect(highlighter.highlightedHTML(for: "plain text", language: nil) == nil)
+        #expect(highlighter.highlightedHTML(for: "plain text", language: "totally-not-a-language") == nil)
+    }
+
+    @Test
+    func producesAttributedTextForNativeViewer() {
+        let highlighter = SyntaxHighlighter(theme: .light)
+        let font = UIFont.monospacedSystemFont(ofSize: 12, weight: .regular)
+        let attributed = highlighter.attributedText(for: "print(\"hi\")", language: "python", font: font)
+
+        let unwrapped = try? #require(attributed)
+        #expect(unwrapped?.string.contains("print") == true)
+    }
+}