fix: boot looping when opening large XTC files (#1648)

Opening XTC files with a high page count (e.g. *The Magic Mountain* at
4,187 pages) causes an immediate `abort()` crash and reboot loop. The
device becomes unusable until the book is removed from the SD card.

**Crash log:**
```
abort() was called at 0x4214a5fb on core 0
```

### Root cause

During `XtcParser::open()`, the parser calls
`m_pageTable.resize(pageCount)` to load the entire page table into RAM.
Each `PageInfo` entry is 16 bytes, so:

- 4,187 pages x 16 bytes = **66,992 bytes (~65KB)** as a single
contiguous heap allocation

On the ESP32-C3 with ~380KB total RAM (no PSRAM), this allocation fails
after firmware, fonts, and the activity system are already loaded.
Because the firmware is compiled with `-fno-exceptions`, the failed
`new` inside `std::vector::resize()` calls `abort()` instead of
throwing.

This affects any XTC file with roughly 3,000+ pages, depending on heap
state at the time of loading.

## Solution

Replace the bulk page table allocation with on-demand reads from the SD
card. Instead of loading all page table entries into a vector at file
open, we now:

1. Read only the **first** page table entry at open time (to get default
page dimensions)
2. Read a **single** 16-byte entry from the SD card each time a page is
loaded

This reduces page table memory usage from `pageCount * 16` bytes to
**zero bytes**, regardless of how many pages the file contains.

### Changes

| File | What changed |
|------|-------------|
| `XtcParser.h` | Removed `std::vector<PageInfo> m_pageTable`. Added
`readPageTableEntry()` for on-demand reads. |
| `XtcParser.cpp` | Replaced `readPageTable()` with
`readFirstPageInfo()`. Updated `getPageInfo()`, `loadPage()`, and
`loadPageStreaming()` to seek and read individual entries from the file.
|

## Trade-offs

### Performance

Each page turn now requires one additional SD card seek + 16-byte read
to look up the page table entry before reading the page data itself.

- SD card sequential read latency: ~0.1-0.5ms for a 16-byte read
- E-ink full display refresh: ~1,000-2,000ms

I personally can't see any performance difference while reading and the
trade off of not boot looping seems to make this well worth it.

### Memory

| Metric | Before | After |
|--------|--------|-------|
| Page table RAM (4,187 pages) | ~65KB | 0 bytes |
| Page table RAM (1,000 pages) | ~16KB | 0 bytes |
| Page table RAM (max 65,535 pages) | ~1MB (impossible) | 0 bytes |

lib/Xtc/Xtc.cpp

@@ -103,7 +103,7 @@ bool Xtc::hasChapters() const {
   return parser->hasChapters();
 }
 
-const std::vector<xtc::ChapterInfo>& Xtc::getChapters() const {
+const std::vector<xtc::ChapterInfo>& Xtc::getChapters() {
   static const std::vector<xtc::ChapterInfo> kEmpty;
   if (!loaded || !parser) {
     return kEmpty;