feat: add backup progress estimation with time remaining

- Add estimate_backup_size() using du -sb before rsync starts
- Parse transfer speed from rsync output (KB/s, MB/s, GB/s)
- Calculate remaining time based on speed and remaining bytes
- Show estimated time remaining and size progress in UI
- Add format_time_remaining() for human-readable time display
- New UI elements: backup_time_label_ and backup_size_label_

src/rsync_manager.h

@@ -5,6 +5,7 @@
 #include <atomic>
 #include <thread>
 #include <mutex>
+#include <chrono>
 #include "config.h"
 
 struct RsyncProgress {
@@ -13,6 +14,9 @@ struct RsyncProgress {
 	int64_t bytes_transferred = 0;
 	int64_t bytes_total = 0;
 	int speed = 0;
+	int64_t estimated_total_size = 0;
+	double elapsed_seconds = 0;
+	double estimated_remaining = 0;
 	std::string current_file;
 	std::string log_line;
 	bool finished = false;
@@ -43,6 +47,9 @@ public:
 
 	std::function<void(const RsyncProgress&)> on_progress;
 
+	int64_t estimate_backup_size(const std::string& source);
+	std::string format_time_remaining(double seconds);
+
 private:
 	Config& config_;
 	std::atomic<bool> running_{false};
@@ -50,6 +57,7 @@ private:
 	std::thread worker_;
 	std::mutex log_mutex_;
 	std::vector<std::string> log_buffer_;
+	std::chrono::steady_clock::time_point start_time_;
 
 	std::vector<std::string> build_exclude_args();
 	bool run_rsync(const std::vector<std::string>& args);