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/EpubReaderFootnotesActivity.cpp

@@ -0,0 +1,95 @@
+#include "EpubReaderFootnotesActivity.h"
+
+#include <GfxRenderer.h>
+#include <I18n.h>
+
+#include <algorithm>
+
+#include "MappedInputManager.h"
+#include "components/UITheme.h"
+#include "fontIds.h"
+
+void EpubReaderFootnotesActivity::onEnter() {
+  ActivityWithSubactivity::onEnter();
+  selectedIndex = 0;
+  requestUpdate();
+}
+
+void EpubReaderFootnotesActivity::onExit() { ActivityWithSubactivity::onExit(); }
+
+void EpubReaderFootnotesActivity::loop() {
+  if (subActivity) {
+    subActivity->loop();
+    return;
+  }
+
+  if (mappedInput.wasReleased(MappedInputManager::Button::Back)) {
+    onGoBack();
+    return;
+  }
+
+  if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
+    if (selectedIndex >= 0 && selectedIndex < static_cast<int>(footnotes.size())) {
+      onSelectFootnote(footnotes[selectedIndex].href);
+    }
+    return;
+  }
+
+  buttonNavigator.onNext([this] {
+    if (!footnotes.empty()) {
+      selectedIndex = (selectedIndex + 1) % footnotes.size();
+      requestUpdate();
+    }
+  });
+
+  buttonNavigator.onPrevious([this] {
+    if (!footnotes.empty()) {
+      selectedIndex = (selectedIndex - 1 + footnotes.size()) % footnotes.size();
+      requestUpdate();
+    }
+  });
+}
+
+void EpubReaderFootnotesActivity::render(Activity::RenderLock&&) {
+  renderer.clearScreen();
+
+  renderer.drawCenteredText(UI_12_FONT_ID, 15, tr(STR_FOOTNOTES), true, EpdFontFamily::BOLD);
+
+  if (footnotes.empty()) {
+    renderer.drawCenteredText(UI_10_FONT_ID, 90, tr(STR_NO_FOOTNOTES));
+    const auto labels = mappedInput.mapLabels(tr(STR_BACK), "", "", "");
+    GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
+    renderer.displayBuffer();
+    return;
+  }
+
+  constexpr int startY = 50;
+  constexpr int lineHeight = 36;
+  const int screenWidth = renderer.getScreenWidth();
+  constexpr int marginLeft = 20;
+
+  const int visibleCount = std::max(1, (renderer.getScreenHeight() - startY) / lineHeight);
+  if (selectedIndex < scrollOffset) scrollOffset = selectedIndex;
+  if (selectedIndex >= scrollOffset + visibleCount) scrollOffset = selectedIndex - visibleCount + 1;
+
+  for (int i = scrollOffset; i < static_cast<int>(footnotes.size()) && i < scrollOffset + visibleCount; i++) {
+    const int y = startY + (i - scrollOffset) * lineHeight;
+    const bool isSelected = (i == selectedIndex);
+
+    if (isSelected) {
+      renderer.fillRect(0, y, screenWidth, lineHeight, true);
+    }
+
+    // Show footnote number and abbreviated href
+    std::string label = footnotes[i].number;
+    if (label.empty()) {
+      label = tr(STR_LINK);
+    }
+    renderer.drawText(UI_10_FONT_ID, marginLeft, y + 4, label.c_str(), !isSelected);
+  }
+
+  const auto labels = mappedInput.mapLabels(tr(STR_BACK), tr(STR_SELECT), "", "");
+  GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
+
+  renderer.displayBuffer();
+}