Add web frontend and refactor core to use OsmError

- Refactor core.py: replace sys.exit() calls with OsmError exceptions
  so the library is safe to use from Flask and other callers
- Add fetch_sibling_routes and fetch_route_master_routes to core.py
- Add Flask web frontend (web/app.py, templates, static assets):
  - Map view with Leaflet; full route drawn in grey on load
  - Sidebar stop list; active-slot UX for From/To selection
  - Segment preview and download (full route or selected segment)
  - Include-stops toggle applied client-side
  - Bookmarkable URLs: GET /<relation_id>
  - Clear selection button
  - Other directions panel (sibling routes from same route_master)
  - route_master handling: draws all member routes in colour on map
    with links to each individual direction
- Add SVG favicon
- Add py.typed marker; add .gitignore

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

src/osm_geojson/pt/cli.py

@@ -6,12 +6,13 @@ import click
 
 from osm_geojson.pt.core import (
     GeoJson,
+    OsmError,
     build_route_coords,
     fetch_relation_full,
     make_geojson,
+    nearest_coord_index,
     node_name,
     parse_elements,
-    nearest_coord_index,
 )
 
 
@@ -35,8 +36,12 @@ def cli() -> None:
 @click.argument("relation_id", type=int)
 def list_stations(relation_id: int) -> None:
     """List all stations in an OSM public transport route relation."""
-    data = fetch_relation_full(relation_id)
-    nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+    try:
+        data = fetch_relation_full(relation_id)
+        nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+    except OsmError as e:
+        click.echo(f"Error: {e}", err=True)
+        sys.exit(1)
     click.echo(f"Route: {tags.get('name', relation_id)}")
     click.echo(f"Stops ({len(stop_ids)}):")
     for i, sid in enumerate(stop_ids, 1):
@@ -60,8 +65,13 @@ def route_between(
     no_stops: bool,
 ) -> None:
     """Output GeoJSON for the route segment between two named stations."""
-    data = fetch_relation_full(relation_id)
-    nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+    try:
+        data = fetch_relation_full(relation_id)
+        nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+    except OsmError as e:
+        click.echo(f"Error: {e}", err=True)
+        sys.exit(1)
+
     route_coords = build_route_coords(way_ids, ways, nodes)
 
     def find_stop(name: str) -> int | None:
@@ -80,8 +90,8 @@ def route_between(
     if sid_to is None:
         errors.append(f"Station not found: {to_station!r}")
     if errors:
-        for e in errors:
-            click.echo(f"Error: {e}", err=True)
+        for msg in errors:
+            click.echo(f"Error: {msg}", err=True)
         click.echo("Available stations:", err=True)
         for sid in stop_ids:
             if sid in nodes:
@@ -104,8 +114,12 @@ def route_between(
 @click.option("--no-stops", is_flag=True, default=False, help="Omit stop points from output.")
 def full_route(relation_id: int, output: str | None, no_stops: bool) -> None:
     """Output GeoJSON for the entire route, end to end."""
-    data = fetch_relation_full(relation_id)
-    nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+    try:
+        data = fetch_relation_full(relation_id)
+        nodes, ways, stop_ids, way_ids, tags = parse_elements(data, relation_id)
+    except OsmError as e:
+        click.echo(f"Error: {e}", err=True)
+        sys.exit(1)
     route_coords = build_route_coords(way_ids, ways, nodes)
     geojson = make_geojson(route_coords, stop_ids, nodes, tags, no_stops=no_stops)
     output_geojson(geojson, output)