perf: drop <sstream> from JsonSettings, freeing 256KB of flash

[socquique]

Problem

JsonSettings::getIntVector() / putIntVector() used std::istringstream and
std::ostringstream to handle a comma separated list of small integers.

Including <sstream> pulls the C++ iostreams and std::locale machinery into the
image, and with it newlib's wide-character and floating point printf/scanf
families. The largest symbols actually linked into the esp32_c3 build:

8.9 KB  _vfprintf_r        8.7 KB  _svfprintf_r      8.6 KB  _svfwprintf_r
8.0 KB  __ssvfscanf_r      6.7 KB  __ssvfiscanf_r    5.3 KB  _vfiprintf_r
5.1 KB  _svfiprintf_r      3.3 KB  _dtoa_r           3.3 KB  _strtod_l
2.4 KB  std::locale::_Impl::_Impl (x2)

None of that is used by this project. _svfwprintf_r is wide-character printf.

Change

Hand-rolled strtol parsing and String concatenation instead.

esp32_c3 firmware.bin   1,299,024 -> 1,042,608 bytes
                            99.1% ->     79.5% of the partition
                                        -256,416 bytes

That is the difference between an image with 11 KB of headroom and one with 268 KB.
The esp32_c3 build was close enough to its partition that adding almost anything
would have overflowed it.

Parsing is equivalent for well formed input; it stops at the first non-numeric
token instead of throwing, so it no longer needs std::stoi's exceptions.

Bounds check

That leniency exposes an existing hole, fixed in the same commit:
SplitFlapDisplay::init() sizes its loops from moduleCount but indexed the
address and offset vectors without checking their length, reading past the end of
the vector for a short or malformed list. Both now fall back to per-module
defaults, and numModules is clamped to MAX_MODULES so it cannot overrun
modules[].

Testing

Parser checked against the real setting values and edge cases — well formed lists,
embedded spaces, trailing comma, empty string, single value. Built and run on an
8 module ESP32-C3 display; addresses and offsets load identically.

Touches JsonSettings.cpp alongside the settings PR; trivial to rebase whichever
lands second.