fix(gps-tracks): rename health fields and fix layer insert priority (F-04, F-05)

F-04: rename gps_health() response fields per tester feedback:
  - total_tracks → tracks_total
  - by_activity  → tracks_by_activity
  - recent_pipeline_runs (list) → last_pipeline_run (object | null)
  Change LIMIT from 10 to 1; fetch single row instead of a list.

F-05: rewrite _findGpsInsertPosition with explicit priority order:
  1. gpx-layer-* (ET-006 GPX file layers) — highest priority
  2. route-* (ET-002 routing layers)
  Remove old combined find() that lacked clear priority semantics.

Add tests/web/gps_tracks.test.js (22 JS unit tests via node:test):
  - _findGpsInsertPosition priority logic (9 cases)
  - Filter state management — default state assertions (5 cases)
  - Color palette mapping and _buildColorExpression (8 cases)

Add tests/web/test_gps_tracks.py — Python pytest runner (8 static
  checks + node --test invocation).

Refs: ET-008

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/api/gps_tracks/endpoint.py

@@ -224,10 +224,11 @@ def create_gps_router(db_path: str) -> APIRouter:
                        status, tracks_new, tracks_updated
                 FROM pipeline_runs
                 ORDER BY started_at DESC
-                LIMIT 10
+                LIMIT 1
                 """
             )
-            recent_runs = [dict(row) for row in cur.fetchall()]
+            last_run_row = cur.fetchone()
+            last_run = dict(last_run_row) if last_run_row else None
 
             cur.execute("SELECT sources_json FROM tracks")
             tracks_by_source: dict = {}
@@ -252,9 +253,9 @@ def create_gps_router(db_path: str) -> APIRouter:
         return {
             "status": "ok",
             "db_path": db_path,
-            "total_tracks": total_tracks,
-            "by_activity": by_activity,
-            "recent_pipeline_runs": recent_runs,
+            "tracks_total": total_tracks,
+            "tracks_by_activity": by_activity,
+            "last_pipeline_run": last_run,
             "db_size_mb": db_size_mb,
             "tracks_by_source": tracks_by_source,
             "tile_cache_size": len(_gps_tile_cache),