Explore: personal GPS heatmap tab under Athlete page

- bincio/explore.py: bake_tracks() simplifies GPS coords (RDP ε=0.0001),
  strips to [lng,lat], groups by sport type, writes per-handle tracks.json
- bake-tracks CLI command; render CLI calls _bake_tracks() after each build;
  strava_zip runs it once at end of batch
- /api/me/tracks endpoint serves the baked file; wipe_user cleans it up
- Explore.svelte: MapLibre full-screen map with sidebar — type pills,
  year/month date filter, Lines / Heatmap (global or by-type) view modes
- AthleteView: Explore tab visible only to profile owner (checks __bincioMe)
- Base.astro: fullscreen prop + Planner nav link

scripts/dev_test.py

@@ -142,6 +142,11 @@ def prepare_serve() -> None:
     _write_root_manifest(DATA_DIR)
     ok("root manifest updated")
 
+    from bincio.explore import bake_tracks
+    for handle in ("dave", "brut"):
+        n = bake_tracks(handle, DATA_DIR)
+        ok(f"{handle}: {n} track(s) baked for explore")
+
 
 # ── 4. Hand off to bincio dev ─────────────────────────────────────────────────