perf(memory): optimize C++ string operations to reduce heap fragmentation

- Replaced pass-by-value std::string parameters with const std::string&
or std::string_view to prevent SSO overallocations.

- Refactored static mapping functions in UPower to return
std::string_view instead of constructing std::string literals, enabling
perfect cache locality.

- Optimized string concatenation in hot loops (network IPs, inhibitor
lists, sway window marks) by using std::string::append() and
pre-reserving capacity instead of overloaded operator+ which produces
temporary heap instances.

These optimizations reduce high-frequency memory churn and overall heap
fragmentation within the main rendering loops.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

test/utils/sleeper_thread.cpp

@@ -4,11 +4,12 @@
 #include <catch2/catch.hpp>
 #endif
 
-#include <chrono>
 #include <sys/wait.h>
-#include <thread>
 #include <unistd.h>
 
+#include <chrono>
+#include <thread>
+
 #include "util/sleeper_thread.hpp"
 
 namespace waybar::util {