Faster web UI: only changed lists reloaded, icon cache kept, storage usage cached

- Web UI: loadLists(only) fetches only the lists an action changed; saving a
  bitmap, icon or font also reloads the pages that use it
- Icons: a save or delete forgets only that name and updates the name list in
  place instead of listing the directory again
- Storage usage cached until a store writes or deletes a file (at most 60 s):
  /api/status 0.6 s -> 0.08 s
- Version 2.19.2

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -227,6 +227,15 @@ See README.md for the overview.
   takes about 100 ms, and pages are drawn for every frame and every web preview. Writes
   must go through `saveBitmap()`/`deleteBitmap()`, which clear the cache. Long JSON lists
   are sent in pieces of about 2 KB (`sendPiece()` in `web_ui.cpp`).
+- Icons are cached too (`display/icon_store`, all of them). A save or delete only forgets
+  that name (`forget()`) and updates the cached name list in place (`listAdd()`/
+  `listRemove()`): clearing it all made the next icon list read every icon again (7 s).
+- Storage usage (`bitmapStorageUsage()`, for `/api/status`) is cached: `usedBytes()` walks
+  the file system (about 0.5 s). Every store calls `bitmapStorageChanged()` after a write
+  or delete; the cache also expires after 60 s.
+- The web UI reloads only the lists that changed (`loadLists(only)`, the ESP32 answers one
+  request at a time): saving a bitmap, icon or font also reloads the pages when a page
+  uses it (`pagesUsing()`, `pagesWithFont()`).
 - Check the page's JavaScript after changes: extract the `<script>` part and run
   `node --check` on it (node is installed).