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/network.cpp

@@ -321,6 +321,7 @@ auto waybar::modules::Network::update() -> void {
   } else if (addr_pref_ == ip_addr_pref::IPV6) {
     final_ipaddr_ = ipaddr6_;
   } else if (addr_pref_ == ip_addr_pref::IPV4_6) {
+    final_ipaddr_.reserve(ipaddr_.length() + ipaddr6_.length() + 1);
     final_ipaddr_ = ipaddr_;
     final_ipaddr_ += '\n';
     final_ipaddr_ += ipaddr6_;