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

README.md

@@ -85,6 +85,14 @@ Terminal=false
    | `Ctrl/Cmd + E` | `inline code` |
    | `Ctrl/Cmd + Shift + X` | ~~strikethrough~~ (`~~…~~`) |
    | `Ctrl/Cmd + K` | link — `[selection](url)`, with `url` selected to replace |
+
+   Press **`Ctrl/Cmd + F`** (or **Edit ▸ Find / Replace…**) to open the
+   find/replace bar above the editor; **`Ctrl/Cmd + H`** opens it with the
+   replace field focused. Type a search term to shade every match in the editor
+   (the current one brighter); **Enter** / **Shift+Enter** (or **▼** / **▲**)
+   step through them, wrapping around, and scroll the match into view. **Aa**
+   toggles case sensitivity. **Replace** swaps the current match and advances;
+   **Replace all** swaps every match in the file at once. **Esc** closes the bar.
 4. Set the **Chapter title** for the open file if you want to override the
    auto-derived one (the field shows the automatic title as a hint). Leaving it
    blank falls back to a `# Title:` line in the header, and if there is none, to