docs: refresh cache formats and web server workflows (#2233)

## Summary

* **What is the goal of this PR?** Update project documentation to match
the current master implementation for cache formats, i18n, file
transfer/web server workflows, SD-card fonts, and root user-facing docs.
* **What changes are included?** Refreshes `book.bin`/`section.bin` docs
for v6/v25, updates File Transfer/Calibre/WebDAV/API docs, documents 24
UI languages and JSON language persistence, updates root
README/USER_GUIDE cache and network details, and syncs the tracked
CLAUDE skill doc cache-version notes.

## Additional Context

* Docs-only change. Verified with `git diff --check origin/master..HEAD`
and stale-reference greps for old cache versions, removed i18n APIs, old
WiFi screen wording, and raw `Serial.printf` examples. No firmware build
was run.

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**NO**_

docs/focus-reading.md

@@ -11,7 +11,7 @@ Focus Reading is a reading aid that bolds the first portion of each word, guidin
 1. Open **Settings > Reader**
 2. Toggle **Focus Reading** on
 
-Toggling the setting will trigger a re-index of your current book, the same as when changing font settings. Once indexing is complete, page turns proceed as normal. No changes are made to your EPUB files.
+Toggling the setting invalidates affected EPUB section caches for the current layout, the same as changing font settings. Sections are rebuilt on demand, then page turns proceed as normal. No changes are made to your EPUB files.
 
 ## Examples