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/disk.cpp

@@ -92,7 +92,7 @@ auto waybar::modules::Disk::update() -> void {
   ALabel::update();
 }
 
-float waybar::modules::Disk::calc_specific_divisor(std::string divisor) {
+float waybar::modules::Disk::calc_specific_divisor(const std::string& divisor) {
   if (divisor == "kB") {
     return 1000.0;
   } else if (divisor == "kiB") {