feat: BookInfo button mapping, ManageBook integration, cover regen fix

- Fix BookInfo buttons: Left/Right front = scroll down/up, Confirm = no-op,
  side buttons retained. Separate Up/Down hints on btn3/btn4.
- Fallback load: try epub.load(true, true) when cache-only load fails,
  so Book Info works for unopened books.
- Add "Book Info" to ManageBook menu (BOOK_INFO action) with handlers in
  all 4 result sites (Home, Recent, FileBrowser, Reader).
- Fix HomeActivity cover regen: call generateCoverBmp(false) + validate
  with isValidThumbnailBmp before falling to placeholder, matching the
  reader's multi-tier fallback pipeline. Same for XTC branch.

Made-with: Cursor

src/activities/home/RecentBooksActivity.cpp

@@ -7,6 +7,7 @@
 #include <algorithm>
 
 #include "../util/ConfirmationActivity.h"
+#include "BookInfoActivity.h"
 #include "BookManageMenuActivity.h"
 #include "MappedInputManager.h"
 #include "RecentBooksStore.h"
@@ -51,6 +52,8 @@ void RecentBooksActivity::onExit() {
 void RecentBooksActivity::executeManageAction(BookManageMenuActivity::Action action, const std::string& capturedPath) {
   bool success = false;
   switch (action) {
+    case BookManageMenuActivity::Action::BOOK_INFO:
+      return;
     case BookManageMenuActivity::Action::ARCHIVE:
       success = BookManager::archiveBook(capturedPath);
       break;
@@ -92,7 +95,11 @@ void RecentBooksActivity::openManageMenu(const std::string& bookPath) {
         const auto& menuResult = std::get<MenuResult>(result.data);
         auto action = static_cast<BookManageMenuActivity::Action>(menuResult.action);
 
-        if (action == BookManageMenuActivity::Action::DELETE || action == BookManageMenuActivity::Action::ARCHIVE) {
+        if (action == BookManageMenuActivity::Action::BOOK_INFO) {
+          startActivityForResult(std::make_unique<BookInfoActivity>(renderer, mappedInput, capturedPath),
+                                 [this](const ActivityResult&) { requestUpdate(); });
+        } else if (action == BookManageMenuActivity::Action::DELETE ||
+                   action == BookManageMenuActivity::Action::ARCHIVE) {
           const char* promptKey =
               (action == BookManageMenuActivity::Action::DELETE) ? tr(STR_DELETE_BOOK) : tr(STR_ARCHIVE_BOOK);
           std::string heading = std::string(promptKey) + "?";