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, 10 insertions, 2 deletions

diff --git a/Hutch/Views/Tickets/TicketDetailView.swift b/Hutch/Views/Tickets/TicketDetailView.swift
index 6339776..1ace6b6 100644
--- a/Hutch/Views/Tickets/TicketDetailView.swift
+++ b/Hutch/Views/Tickets/TicketDetailView.swift
@@ -553,14 +553,22 @@ private struct MarkdownContentView: View {
                     .frame(minHeight: 80)
             }
         }
-        .task(id: markdown) {
+        .task(id: MarkdownRenderKey(markdown: markdown, isDark: colorScheme == .dark)) {
+            let theme = SyntaxHighlightTheme(colorScheme: colorScheme)
             let html = await Task.detached(priority: .userInitiated) {
-                markdownToHTML(markdown)
+                markdownToHTML(markdown, codeTheme: theme)
             }.value
             guard !Task.isCancelled else { return }
             renderedHTML = html
         }
     }
+
+    /// Re-renders when the markdown or the color scheme changes; highlighted
+    /// code colors are baked into the HTML, so both must key the render.
+    private struct MarkdownRenderKey: Equatable {
+        let markdown: String
+        let isDark: Bool
+    }
 }
 
 // MARK: - Event Row