fix: several bookmarks UX improvments (#2372)

## Summary

This PR enhances the EPUB reader's bookmark system with two
complementary improvements: a per-page bookmark indicator icon and
toggle behavior on the existing long-press action.

---

### What Changed

**Bookmark Toggle (was: add-only)**

The long-press Confirm action now toggles bookmarks rather than always
adding. `addBookmark()` checks whether a bookmark with the same xpath
already exists in the in-memory cache:
- If found → removes it and shows "Bookmark removed."
- If not found → adds it and shows "Bookmark added."

A new `STR_BOOKMARK_REMOVED` translation string was added to support the
removal message.

**Bookmark Icon Indicator**

A `BookmarkIcon` is now drawn at the top-right corner of the page
whenever the current page has a bookmark. `updateBookmarkFlag()` is
called at render time to determine whether the current page is
bookmarked.

**In-Memory Bookmark Cache**

Bookmarks are now loaded into `cachedBookmarks` on `onEnter()` rather
than being re-read from disk on every toggle. All subsequent add/remove
operations work against this cache and flush to disk, avoiding redundant
file reads on each bookmark action.

**Faster bookmarks list**

Previously, calculating "page X/Y" for each entry required decompressing
the entire spine item. We now persist `si`/`pc`/`pp` (spine index, page
count, and page progress) in the bookmark JSON when saving, and restore
them when loading. This avoids the expensive `toCrossPoint()` loop in
`onEnter()`, significantly reducing the cost of initializing the
bookmarks list.

---

### Files Changed

- `EpubReaderActivity.cpp` — `addBookmark()` toggle logic,
`updateBookmarkFlag()` (new), icon rendering in `renderContents()`,
cache initialization in `onEnter()`
- `EpubReaderActivity.h` — new fields: `currentPageBookmarked`,
`bookmarkRemoved`, `cachedBookmarks`; new method declaration
`updateBookmarkFlag()`

---

### 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? _**< PARTIALLY >**_

---------

Co-authored-by: Julia Nguyen <julia@uxj.io>

lib/I18n/translations/slovenian.yaml

@@ -184,6 +184,9 @@ STR_HOME: "« Domov"
 STR_SELECT: "Izberi"
 STR_SELECTED: "Izbrano"
 STR_TOGGLE: "Preklopi"
+STR_TOGGLE_BOOKMARK: "Preklopi zaznamek"
+STR_BOOKMARK_REMOVED: "Zaznamek odstranjen."
+STR_HOLD_OPEN_TO_DELETE: "Držite Odpri za brisanje"
 STR_CONFIRM: "Potrdi"
 STR_CANCEL: "Prekliči"
 STR_CONNECT: "Poveži"