Fix title truncation crash for short titles (#63)

## Problem
The status bar title truncation loop crashes when the chapter title is
shorter than 8 characters.

```cpp
// title.length() - 8 underflows when length < 8 (size_t is unsigned)
title = title.substr(0, title.length() - 8) + "...";
```

## Fix
Added a length guard to skip truncation for titles that are too short to
truncate safely.

## Testing
- Builds successfully with `pio run`
- Affects: `src/activities/reader/EpubReaderActivity.cpp`

src/activities/reader/EpubReaderActivity.cpp

@@ -382,7 +382,7 @@ void EpubReaderActivity::renderStatusBar() const {
     const auto tocItem = epub->getTocItem(tocIndex);
     title = tocItem.title;
     titleWidth = renderer.getTextWidth(SMALL_FONT_ID, title.c_str());
-    while (titleWidth > availableTextWidth) {
+    while (titleWidth > availableTextWidth && title.length() > 11) {
       title = title.substr(0, title.length() - 8) + "...";
       titleWidth = renderer.getTextWidth(SMALL_FONT_ID, title.c_str());
     }