Places map: city base map, cluster gallery, zoom + render polish

Base map / tiles:
- Switch the base map to the mapchart city-label world map; re-derive its
  Web-Mercator georeferencing (MAP_LON0/1, MAP_IMG_MY0/1) by least-squares
  against the pixel-identical graticule export.
- Add a 1.5x LANCZOS supersample LOD and bump tile quality 90->94 so city
  labels stay crisp when zoomed in (tools/png_to_tiles.py).
- Fill the map view background with the ocean colour instead of black.

Navigation / clustering:
- Snap the cluster grid to power-of-two zoom octaves so the bubbles stop
  jittering during a continuous zoom.
- Replace the loose zoom clamps with named MAP_SCALE_MIN/MAX (whole map fills
  the screen .. country/region level).
- Add a toggleable computed lat/lon graticule overlay (TRIANGLE).

Cluster gallery:
- Selecting a bubble now opens all its photos on the same justified grid as
  the timeline/search (a g_gallery_active filter). Members not already paged
  in are appended as hidden g_asset entries (kept out of the main timeline);
  a background metadata worker resolves each one's date/aspect so the grid
  reflows into month sections + true aspect ratios.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011uKJQMYDE9Nrq9ECsU3MxH

src/main.c

@@ -128,6 +128,9 @@ static unsigned char *g_asset_is_video;
 /* set during display rebuild: a backed-up local file matches this server
  * asset, so its grid cell shows the green "backed up" badge */
 static unsigned char *g_asset_local_backed;
+/* 1 = appended only to back a map-cluster gallery (its id/thumb are needed but
+ * it isn't part of the paged library): shown in the gallery, hidden elsewhere */
+static unsigned char *g_asset_hidden;
 static int g_asset_count;
 static int g_asset_cap;
 static int g_next_page = 1; /* 0 = no more pages */