## Problem
Three Epub getter functions can throw exceptions:
- `getCumulativeSpineItemSize()`: No bounds check before
`.at(spineIndex)`
- `getSpineItem()`: If spine is empty and index invalid, `.at(0)` throws
- `getTocItem()`: If toc is empty and index invalid, `.at(0)` throws
## Fix
- Add bounds check to `getCumulativeSpineItemSize()`, return 0 on error
- Add empty container checks to `getSpineItem()` and `getTocItem()`
- Use static fallback objects for safe reference returns on empty
containers
Changed `lib/Epub/Epub.cpp`.
## Test
- Defensive additions - follows existing bounds check patterns
- No logic changes for valid inputs
- Manual device testing appreciated
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