fix: unique process keys per instance to prevent collision on multi-monitor setups

[navidagz]

Description

Problem

When running two instances of the plugin (one per monitor) and calling dms ipc call plugins reload pkgUpdate,
one instance would get stuck showing "Checking for updates" indefinitely.

The cause: Proc.runCommand uses a string key to identify and deduplicate managed processes globally across the
shell. All three keys (pkgUpdate.dnf, pkgUpdate.flatpakMeta, pkgUpdate.flatpakUpdates) were hardcoded, so when
the second instance called checkUpdates(), DMS saw those keys already in use by the first instance and skipped
spawning new processes. The callbacks never fired, leaving dnfChecking and flatpakChecking stuck at true.

Fix

Added a _procNs property that generates a random per-instance prefix at construction time using
Math.random().toString(36).slice(2). All Proc.runCommand keys are now namespaced under this prefix, making them
unique per instance regardless of how many bars are running.

Verification

Confirmed two instances produce distinct keys on reload:

procNs = pkgUpdate.f2yggwctnyinj025t5qj8a.dnf
procNs = pkgUpdate.43suodu04i5f08dplmb5oe.dnf

[navidagz]

@rahulmysore23