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/EpubReaderActivity.h

@@ -1,5 +1,6 @@
 #pragma once
 #include <Epub.h>
+#include <Epub/FootnoteEntry.h>
 #include <Epub/Section.h>
 
 #include "EpubReaderMenuActivity.h"
@@ -25,6 +26,16 @@ class EpubReaderActivity final : public ActivityWithSubactivity {
   const std::function<void()> onGoBack;
   const std::function<void()> onGoHome;
 
+  // Footnote support
+  std::vector<FootnoteEntry> currentPageFootnotes;
+  struct SavedPosition {
+    int spineIndex;
+    int pageNumber;
+  };
+  static constexpr int MAX_FOOTNOTE_DEPTH = 3;
+  SavedPosition savedPositions[MAX_FOOTNOTE_DEPTH] = {};
+  int footnoteDepth = 0;
+
   void renderContents(std::unique_ptr<Page> page, int orientedMarginTop, int orientedMarginRight,
                       int orientedMarginBottom, int orientedMarginLeft);
   void renderStatusBar() const;
@@ -35,6 +46,10 @@ class EpubReaderActivity final : public ActivityWithSubactivity {
   void onReaderMenuConfirm(EpubReaderMenuActivity::MenuAction action);
   void applyOrientation(uint8_t orientation);
 
+  // Footnote navigation
+  void navigateToHref(const char* href, bool savePosition = false);
+  void restoreSavedPosition();
+
  public:
   explicit EpubReaderActivity(GfxRenderer& renderer, MappedInputManager& mappedInput, std::unique_ptr<Epub> epub,
                               const std::function<void()>& onGoBack, const std::function<void()>& onGoHome)