fix(epub): decode percent-encoded internal asset paths so assets render correctly (#2249)

## Summary

* **What is the goal of this PR?** 
* Fix EPUBs where internal file references are written in URL-style
escaped form, like spaces appearing as `%20`, so the reader can find the
right files instead of treating those references as missing.
* **What changes are included?**
* Added a small shared helper that converts those escaped EPUB-internal
paths back into their normal filenames before we try to look them up.
* Applied that cleanup step across the EPUB parsing flow wherever we
resolve internal references, including cover images, manifest items, TOC
links, spine entries, and inline HTML images.
* Kept the change narrowly focused on EPUB-internal asset resolution
rather than changing broader URL or networking behavior.

## Additional Context

* The user-facing bug here is that some books package their internal
filenames in an escaped form, so a file like `Chapter 1.xhtml` may be
referenced more like `Chapter%201.xhtml`. The reader was treating that
escaped text as the literal filename, which means otherwise-valid books
could lose images, covers, or chapter targets because the lookup no
longer matched the real file inside the EPUB.
* Risk is intentionally low. The helper only rewrites valid `%XX` escape
sequences and leaves malformed input alone, so it should improve
compatibility with escaped filenames without broadening the parser’s
behavior in unrelated cases.

## Local Testing Performed
* This was tested on my device with the user-provided optimized epub
that was not rendering images within the text prior to this fix (cover
image and chapter headers were rendering fine):

[orv_main_baseline.epub.zip](https://github.com/user-attachments/files/28529545/orv_main_baseline.epub.zip)
* This was also tested by the user with a local build and the affected
epub and confirmed to be working

## Steps for Testing
* Try to open the affected epub (linked above) or any epub that has
similar percent-encoding on a build prior to this fix.
* Apply this fix, clear book cache, and re-open the affected book.
* Images should render properly.
---

### AI Usage

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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? _**< YES >**_

lib/FsHelpers/FsHelpers.cpp

@@ -7,6 +7,34 @@
 
 namespace FsHelpers {
 
+namespace {
+bool isHexDigit(const char c) { return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F'); }
+
+uint8_t hexValue(const char c) {
+  if (c >= '0' && c <= '9') return static_cast<uint8_t>(c - '0');
+  if (c >= 'a' && c <= 'f') return static_cast<uint8_t>(10 + (c - 'a'));
+  return static_cast<uint8_t>(10 + (c - 'A'));
+}
+}  // namespace
+
+std::string decodeUriEscapes(const std::string& path) {
+  std::string decoded;
+  decoded.reserve(path.size());
+
+  for (size_t i = 0; i < path.size(); i++) {
+    if (path[i] == '%' && i + 2 < path.size() && isHexDigit(path[i + 1]) && isHexDigit(path[i + 2])) {
+      const uint8_t value = static_cast<uint8_t>((hexValue(path[i + 1]) << 4) | hexValue(path[i + 2]));
+      decoded += static_cast<char>(value);
+      i += 2;
+      continue;
+    }
+
+    decoded += path[i];
+  }
+
+  return decoded;
+}
+
 std::string normalisePath(const std::string& path) {
   std::vector<std::string> components;
   std::string component;