feat(countries): replace Map view with browse-by-country (like Tags)

Removes the maplibre-gl Map view and adds "Countries" as a sixth
TagCategory, reusing the existing /tags/[category]/[[value]] browse
machinery instead of a bespoke map UI. Backed by a new self-contained
sidecar endpoint that aggregates photos.photo_country with BasePath
scoping, mirroring handleLabels/handleScopedCounts.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sidecar/main.go

@@ -104,6 +104,7 @@ func main() {
 						if ppDb != nil {
 							auth.GET("/labels", handleLabels(pp, ppDb))
 							auth.GET("/counts", handleScopedCounts(ppDb))
+							auth.GET("/countries", handleCountries(ppDb))
 						}
 
 						// User-scoped photos — post-filters by BasePath so review/archive