Highlight TUI content previews

TESTING.md

@@ -47,9 +47,10 @@ representative repositories.
 - [ ] The five numbered panes open Home, Issues, Repos, PRs, and Milestones;
       the selected pane and row remain visibly distinct at narrow and wide
       terminal sizes.
-- [ ] `j`/`k`, arrows, `g`/`G`, Page Up/Down, Enter, Backspace, and `q` provide
-      mutt-style list and history navigation; `n`/`p` and scrolling past a
-      boundary traverse paginated API results without losing history.
+- [ ] `j`/`k`, arrows, `g`/`G`, Page Up/Down, Enter, and Backspace provide
+      mutt-style list and history navigation; `q` exits immediately from every
+      non-editor depth. `n`/`p` and scrolling past a boundary traverse
+      paginated API results without losing history.
 - [ ] Clicking a pane switches it, clicking a list row selects it,
       double-clicking opens it, and the mouse wheel scrolls the list.
 - [ ] Home activity filters cycle with `v`; activity targets route to their
@@ -57,6 +58,12 @@ representative repositories.
 - [ ] Repository favorites toggle with `*`, sort first per pane, and persist
       after relaunch. Commits switch branches with `b`; file trees, text and
       binary files, commit changes, pull changes, and per-file diffs open.
+- [ ] Source files use grammar-based syntax colors selected by filename or
+      shebang, with unknown text remaining legible. Commit and pull diffs show
+      metadata, hunks, additions, and deletions in distinct syntax colors.
+- [ ] Home activity content plus issue, pull request, milestone, and comment
+      Markdown pseudo-renders headings, emphasis, strong text, lists, links,
+      inline code, and fenced code while preserving metadata line breaks.
 - [ ] Issue and pull filters apply. Issues can be created, edited,
       closed/reopened, and deleted after confirmation; issue comments can be
       added and the current user's comments edited.