feat: show loading popup in BookInfo when parsing unopened book

When BookInfo is opened for a book with no existing cache,
epub.load(true, true) triggers a 1-2s full parse. Show a
"Loading..." popup with progress bar so the device doesn't
appear frozen. Popup only appears on the fallback path —
cached books load silently. Same pattern for XTC books.

Made-with: Cursor

chat-summaries/2026-03-09_03-15-summary.md

@@ -0,0 +1,23 @@
+# BookInfo Loading Popup for Unopened Books
+
+**Date**: 2026-03-09
+**Task**: Show a progress popup when BookInfo needs to parse an unopened book
+
+## Changes Made
+
+### BookInfoActivity.cpp
+
+Added a "Loading..." progress popup with progress bar to `onEnter()` when the book cache doesn't exist and a full parse is required:
+
+- **EPUB branch**: `epub.load(false, true)` is tried first (fast, cache-only). If it fails, `GUI.drawPopup(tr(STR_LOADING))` is shown at 10%, `epub.load(true, true)` runs (slow full parse), progress updates to 50%, thumbnail generation runs, then progress hits 100%.
+- **XTC branch**: Checks `Storage.exists(xtc.getCachePath())` before `xtc.load()`. If cache directory is missing, shows the same popup pattern around load + thumbnail generation.
+- For books with existing cache, no popup is shown — the fast path completes silently.
+
+## Build Result
+
+Build succeeded — 0 errors, 0 linter warnings. RAM: 30.3%, Flash: 95.7%.
+
+## Follow-up
+
+- Test on device: open BookInfo for a book that has never been opened, verify popup appears
+- Verify no popup appears for books that have been previously opened/cached