Add a find/replace bar to the editor

Ctrl+F opens a find bar above the editor (Ctrl+H focuses the replace
field; Edit ▸ Find / Replace… opens it too). Matches are shaded in the
editor with the active one highlighted more strongly; Enter/Shift+Enter
and ▼/▲ step through them with wraparound and scroll-to-view. An "Aa"
toggle controls case sensitivity. Replace swaps the current match and
advances; Replace all swaps every match in one pass. Esc closes the bar.

Search/replace logic lives in pure, unit-tested functions (find_matches,
replace_all) with non-overlapping matching and correct multibyte offset
mapping for case-insensitive search. Highlighting reuses the editor's
layout-job builder alongside the grammar underlines.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N9kRuP7JvXoUGdNNeg5ZSs

src/help.rs

@@ -64,12 +64,16 @@ fn cheatsheet_body(ui: &mut egui::Ui) {
             ("Ctrl/Cmd + E", "Inline code (`…`)."),
             ("Ctrl/Cmd + Shift + X", "Strikethrough (~~…~~)."),
             ("Ctrl/Cmd + K", "Wrap as a link and select the url to replace."),
+            ("Ctrl/Cmd + F", "Open the find bar."),
+            ("Ctrl/Cmd + H", "Open find/replace (replace field focused)."),
         ],
     );
     note(
         ui,
-        "Applied to the selected text (or the caret, for an empty selection). \
-         Pressing the same key on already-wrapped text removes the markers.",
+        "Wrapping keys apply to the selected text (or the caret, for an empty \
+         selection); pressing the same key on already-wrapped text removes the \
+         markers. In the find bar, Enter / Shift+Enter step through matches and \
+         Esc closes it.",
     );
 
     section(ui, "Paragraphs & breaks");