fix(mining): stamp hashrate history and label the chart by real time

The hashrate sparkline guessed the age of its oldest sample from the
sample count, labelling the left edge "5m ago", "1m ago" or "start"
regardless of how fast samples actually arrived. Solo and pool samples
come in at different intervals, and a slow or stalled poll made those
labels wrong.

- Each hashrate sample now carries a wall-clock timestamp, kept in
  lockstep with the value as the history fills and rolls over, for both
  the solo and pool histories.
- The chart's left-edge label is derived from the oldest sample's actual
  timestamp, falling back to "start" only when no stamp is available.

Tests: a mining refresh records a timestamp per sample, and the
timestamp history stays the same length as the value history once the
history is capped.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/data/wallet_state.h

@@ -146,7 +146,8 @@ struct MiningInfo {
     
     // History for chart
     std::vector<double> hashrate_history;  // Last N samples
-    static constexpr int MAX_HISTORY = 300; // 5 minutes at 1s intervals
+    std::vector<std::time_t> hashrate_history_t;  // wall-clock stamp per sample (lockstep with hashrate_history)
+    static constexpr int MAX_HISTORY = 300; // 5 minutes at ~1s intervals
 
     // Recent daemon log lines for the mining log panel
     std::vector<std::string> log_lines;
@@ -263,7 +264,8 @@ struct PoolMiningState {
 
     // Hashrate history for chart (mirrors MiningInfo::hashrate_history)
     std::vector<double> hashrate_history;
-    static constexpr int MAX_HISTORY = 60; // 5 minutes at ~5s intervals
+    std::vector<std::time_t> hashrate_history_t;  // wall-clock stamp per sample (lockstep with hashrate_history)
+    static constexpr int MAX_HISTORY = 60; // ~2 min at the ~2s pool poll
 
     // Recent log lines for the log panel
     std::vector<std::string> log_lines;