feat: slim footnotes support (#1031)

## Summary
**What is the goal of this PR?** Implement support for footnotes in epub
files.
It is based on #553, but simplified — removed the parts which
complicated the code and burden the CPU/RAM. This version supports basic
footnotes and lets the user jump from location to location inside the
epub.

**What changes are included?**
- `FootnoteEntry` struct — A small POD struct (number[24], href[64])
shared between parser, page storage, and UI.
- Parser: `<a href>` detection (`ChapterHtmlSlimParser`) — During a
single parsing pass, internal epub links are detected and collected as
footnotes. The link text is underlined to hint navigability.
Bracket/whitespace normalization is applied to the display label (e.g.
[1] → 1).
- Footnote-to-page assignment (`ChapterHtmlSlimParser`, `Page`) —
Footnotes are attached to the exact page where their anchor word
appears, tracked via a cumulative word counter during layout, surviving
paragraph splits and the 750-word mid-paragraph safety flush.
- Page serialization (`Page`, `Section`) — Footnotes are
serialized/deserialized per page (max 16 per page). Section cache
version bumped to 14 to force a clean rebuild.
- Href → spine resolution (`Epub`) — `resolveHrefToSpineIndex()` maps an
href (e.g. `chapter2.xhtml#note1`) to its spine index by filename
matching.
- Footnotes menu + activity (`EpubReaderMenuActivity`,
`EpubReaderFootnotesActivity`) — A new "Footnotes" entry in the reader
menu lists all footnote links found on the current page. The user
scrolls and selects to navigate.
- Navigate & restore (`EpubReaderActivity`) — `navigateToHref()` saves
the current spine index and page number, then jumps to the target. The
Back button restores the saved position when the user is done reading
the footnote.

  **Additional Context**

**What was removed vs #553:** virtual spine items
(`addVirtualSpineItem`, `isVirtualSpineItem`), two-pass parsing,
`<aside>` content extraction to temp HTML files, `<p class="note">`
paragraph note extraction, `replaceHtmlEntities` (master already has
`lookupHtmlEntity`), `footnotePages` / `buildFilteredChapterList`,
`noterefCallback` / `Noteref` struct, and the stack size increase from 8
KB to 24 KB (not needed without two-pass parsing and virtual file I/O on
the render task).
 
**Performance:** Single-pass parsing. No new heap allocations in the hot
path — footnote text is collected into fixed stack buffers (char[24],
char[64]). Active runtime memory is ~2.8 KB worst-case (one page × 16
footnotes × 88 bytes, mirrored in `currentPageFootnotes`). Flash usage
is unchanged at 97.4%; RAM stays at 31%.
   
**Known limitations:** When clicking a footnote, it jumps to the start
of the HTML file instead of the specific anchor. This could be
problematic for books that don't have separate files for each footnote.
(no element-id-to-page mapping yet - will be another PR soon).

---

### AI Usage

Did you use AI tools to help write this code? _**< PARTIALLY>**_
Claude Opus 4.6 was used to do most of the migration, I checked manually
its work, and fixed some stuff, but I haven't review all the changes
yet, so feedback is welcomed.

---------

Co-authored-by: Arthur Tazhitdinov <lisnake@gmail.com>

src/activities/reader/EpubReaderMenuActivity.h

@@ -14,6 +14,7 @@ class EpubReaderMenuActivity final : public ActivityWithSubactivity {
   // Menu actions available from the reader menu.
   enum class MenuAction {
     SELECT_CHAPTER,
+    FOOTNOTES,
     GO_TO_PERCENT,
     ROTATE_SCREEN,
     SCREENSHOT,
@@ -25,16 +26,9 @@ class EpubReaderMenuActivity final : public ActivityWithSubactivity {
 
   explicit EpubReaderMenuActivity(GfxRenderer& renderer, MappedInputManager& mappedInput, const std::string& title,
                                   const int currentPage, const int totalPages, const int bookProgressPercent,
-                                  const uint8_t currentOrientation, const std::function<void(uint8_t)>& onBack,
-                                  const std::function<void(MenuAction)>& onAction)
-      : ActivityWithSubactivity("EpubReaderMenu", renderer, mappedInput),
-        title(title),
-        pendingOrientation(currentOrientation),
-        currentPage(currentPage),
-        totalPages(totalPages),
-        bookProgressPercent(bookProgressPercent),
-        onBack(onBack),
-        onAction(onAction) {}
+                                  const uint8_t currentOrientation, const bool hasFootnotes,
+                                  const std::function<void(uint8_t)>& onBack,
+                                  const std::function<void(MenuAction)>& onAction);
 
   void onEnter() override;
   void onExit() override;
@@ -47,15 +41,11 @@ class EpubReaderMenuActivity final : public ActivityWithSubactivity {
     StrId labelId;
   };
 
-  // Fixed menu layout (order matters for up/down navigation).
-  const std::vector<MenuItem> menuItems = {{MenuAction::SELECT_CHAPTER, StrId::STR_SELECT_CHAPTER},
-                                           {MenuAction::ROTATE_SCREEN, StrId::STR_ORIENTATION},
-                                           {MenuAction::GO_TO_PERCENT, StrId::STR_GO_TO_PERCENT},
-                                           {MenuAction::SCREENSHOT, StrId::STR_SCREENSHOT_BUTTON},
-                                           {MenuAction::DISPLAY_QR, StrId::STR_DISPLAY_QR},
-                                           {MenuAction::GO_HOME, StrId::STR_GO_HOME_BUTTON},
-                                           {MenuAction::SYNC, StrId::STR_SYNC_PROGRESS},
-                                           {MenuAction::DELETE_CACHE, StrId::STR_DELETE_CACHE}};
+  static std::vector<MenuItem> buildMenuItems(bool hasFootnotes);
+
+  // Fixed menu layout
+  const std::vector<MenuItem> menuItems;
+
   int selectedIndex = 0;
 
   ButtonNavigator buttonNavigator;