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>

src/modules/mpd/mpd.cpp

@@ -239,7 +239,8 @@ std::string waybar::modules::MPD::getStateIcon() const {
   }
 }
 
-std::string waybar::modules::MPD::getOptionIcon(std::string optionName, bool activated) const {
+std::string waybar::modules::MPD::getOptionIcon(const std::string& optionName,
+                                                bool activated) const {
   if (!config_[optionName + "-icons"].isObject()) {
     return "";
   }