fix elevation_gain_m null for modern Garmin FIT files; fix map flash

FIT parser: try enhanced_altitude before altitude. Barometric altimeters
on modern Garmins (Edge 540, 840, etc.) write enhanced_altitude in
record messages and total_ascent in lap messages. The old code read only
altitude, producing null elevation_m per point → null elevation_gain_m
at the activity root while laps had correct values from total_ascent.

ActivityMap: use preview_coords (passed from ActivitySummary) to
initialise the map at the activity's location on mount, eliminating the
flash of world-view before the async detail JSON / bbox arrives.

bincio/extract/parsers/fit.py

@@ -57,12 +57,17 @@ class FitParser:
                     lat = _semicircles_to_deg(_get(frame, "position_lat"))
                     lon = _semicircles_to_deg(_get(frame, "position_long"))
                     speed_raw = _get(frame, "speed")  # m/s
+                    # enhanced_altitude is written by barometric altimeters (most
+                    # modern Garmins). Fall back to GPS-derived altitude if absent.
+                    _alt = _get(frame, "enhanced_altitude")
+                    if _alt is None:
+                        _alt = _get(frame, "altitude")
 
                     dp = DataPoint(
                         timestamp=ts,
                         lat=lat,
                         lon=lon,
-                        elevation_m=_get(frame, "altitude"),
+                        elevation_m=_alt,
                         hr_bpm=_get(frame, "heart_rate"),
                         cadence_rpm=_get(frame, "cadence"),
                         speed_kmh=speed_raw * 3.6 if speed_raw is not None else None,