gui: initialize the SDK on the primary instance only, and fix the resolv.conf log line

[Ryanmello07]

Two independent fixes, both found by reading a tester's log bundle rather than by
reading the code.

1. SDK init ran on every launch, including duplicates

SdkHost::Initialize() was called before app->run(). GApplication only decides
primary-vs-remote inside run(), so that code executes in every process that
starts — including a second launch that is about to hand its arguments to the
running instance and exit.

That is not a harmless double-init. Initialize() calls urnet::setLogDir(),
which runs the SDK's glog init: it sweeps the log directory down to a keep-N
budget and rewrites the urnetwork-gui.{INFO,WARNING,ERROR} symlinks. In the
bundle, a second launch deleted three of the seven log files and left
urnetwork-gui.INFO — the file every "grab the current log" instruction follows
— naming its own 846-byte stub, while the session that was actually running had a
1.5 MB log the symlink no longer pointed at. Anyone diagnosing from that log was
reading the wrong file. Initialize() also opens the shared storage directory,
which a process about to exit has no business touching.

Moved into signal_startup, which is emitted on the primary instance only. A
failure there calls app->quit() instead of return 1, because by that point
main() has already handed control to run().

2. The resolv.conf probe printed a self-referential symlink

The log line rendered <path> -> <realpath> unconditionally. On Arch and CachyOS
/etc/resolv.conf is commonly a regular file that systemd-resolved writes in
place rather than a symlink into its runtime directory, so realpath() returns
the path itself and the line came out as

/etc/resolv.conf -> /etc/resolv.conf (resolved's)

which reads as a symlink loop and sent a tester's triage chasing a misdetection
that did not exist. The arrow is now rendered only when the path actually
resolves somewhere else; a regular file and a missing file each say so
explicitly.

DNS tier selection was always correct — only this line was wrong. No behaviour
change, only the diagnostic.

Why this is split this way

Neither fix has anything to do with the app id, the icon, packaging or Flathub.
They are grouped only because they are both small runtime-correctness fixes found
in the same testing pass, and both are worth landing without waiting for the
larger PRs.

app/src/main.cpp is also touched by PR 2 (the app-id string, ~8 lines above the
hunk here). They merged cleanly in testing; if git ever disagrees, keep both.