## 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
## Problem
- `getBookSize()` calls `getCumulativeSpineItemSize(getSpineItemsCount()
- 1)` which passes -1 when spine is empty
- `calculateProgress()` then divides by zero when book size is 0
## Fix
- Return 0 from `getBookSize()` if spine is empty
- Return 0 from `calculateProgress()` if book size is 0
## Testing
- Builds successfully with `pio run`
- Affects: `lib/Epub/Epub.cpp`
## Problem
`getSpineIndexForTocIndex()` and `getTocIndexForSpineIndex()` access
`toc[tocIndex]` and `spine[spineIndex]` without validating indices are
within bounds. Malformed EPUBs or edge cases could trigger out-of-bounds
access.
## Fix
Added bounds validation at the start of both functions before accessing
the arrays.
## Testing
- Builds successfully with `pio run`
- Affects: `lib/Epub/Epub.cpp`
## Summary
* Rely on media-type="application/x-dtbncx+xml" to find TOC instead of
hardcoded values
## Additional Context
* Most of my epubs don't have id==ncx for toc file location. I think
this media-type is EPUB standard
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
