Move the session-skip from a before filter to real Rack middleware

The before filter added in the previous commit never ran for static
assets: Sinatra serves public_folder files (every JS/CSS/font/vendor
file) through its own built-in shortcut, bypassing route and filter
dispatch entirely — confirmed live, curl still got a fresh set-cookie
on /js/map.js after that commit deployed, and the user hit the exact
same logout on refresh again. A HAR from that retest shows the same
clobbering pattern as before, just via a different response each time.

SkipSessionWrite is a plain Rack middleware instead, positioned (via
`use`, after `set :sessions`) inside Rack::Session::Cookie's own
wrapping — it sees the same env static dispatch does, so every response
is covered, not just Sinatra-routed ones.

app.rb

@@ -15,6 +15,40 @@ require_relative "lib/maptool/plugins"
 require_relative "lib/maptool/pmtiles_builder"
 
 module Maptool
+  # Only /login, /auth/callback and /logout ever write to session
+  # (oauth_state, user, session.clear) — everything else only ever reads
+  # it, if it touches session at all. That still matters:
+  # Rack::Session::Cookie re-serializes and re-sets the cookie on *every*
+  # response that so much as read session (Rack::Protection's own CSRF
+  # check does, on nearly every request) — reflecting whatever cookie
+  # that particular request happened to arrive with, whether or not
+  # anything changed. A HAR capture caught the actual failure mode this
+  # caused: seconds after a successful login, a background poll fired the
+  # instant the page loads was still carrying the browser's pre-login
+  # cookie (a client-side timing race across the OAuth redirect chain,
+  # not something this server controls), and its own re-issued Set-Cookie
+  # landed *after* the login's, silently overwriting session[:user] with
+  # nothing logged anywhere to explain it.
+  #
+  # A plain Rack middleware, not a Sinatra `before` filter: static assets
+  # (public_folder — every JS/CSS/font/vendor file) are served by
+  # Sinatra's own built-in static-file shortcut, which bypasses route and
+  # filter dispatch entirely but is still wrapped by this, same as every
+  # other response — each one was independently capable of winning the
+  # clobbering race a `before` filter alone left uncovered.
+  class SkipSessionWrite
+    WRITE_PATHS = %w[/login /auth/callback /logout].freeze
+
+    def initialize(app)
+      @app = app
+    end
+
+    def call(env)
+      env["rack.session.options"][:skip] = true unless WRITE_PATHS.include?(env["PATH_INFO"])
+      @app.call(env)
+    end
+  end
+
   class App < Sinatra::Base
     set :public_folder, File.join(__dir__, "public")
     set :views, File.join(__dir__, "views")
@@ -33,6 +67,7 @@ module Maptool
     # don't survive a restart there, which is fine outside production.
     set :sessions, httponly: true, secure: production?, same_site: :lax,
                    secret: ENV.fetch("SESSION_SECRET") { SecureRandom.hex(32) }
+    use SkipSessionWrite
 
     # Cache-busts /js and /css requests: browsers (and a caching proxy in
     # front of production, see docs/deployment.adoc) hang onto static
@@ -150,28 +185,6 @@ module Maptool
       end
     end
 
-    # Only these three routes ever write to session (oauth_state, user,
-    # session.clear below) — every other route only ever reads it, if it
-    # touches session at all. That still matters: Rack::Session::Cookie
-    # re-serializes and re-sets the cookie on *every* response that so
-    # much as read session (Rack::Protection's own CSRF check does, on
-    # nearly every request) — reflecting whatever cookie that particular
-    # request happened to arrive with, whether or not anything changed.
-    # A HAR capture caught the actual failure mode this caused: seconds
-    # after a successful login, offline_data.js's _checkUpdate — an
-    # unrelated background poll fired the instant the page loads — was
-    # still carrying the browser's pre-login cookie (a client-side timing
-    # race across the OAuth redirect chain, not something this server
-    # controls), and its own re-issued Set-Cookie landed *after* the
-    # login's, silently overwriting session[:user] back out with nothing
-    # logged anywhere to explain it. Skipping the write everywhere except
-    # the routes that actually need it removes every other response from
-    # that race — a stale request can still be served from a stale
-    # cookie, but it can no longer clobber a fresher one.
-    before do
-      session.options[:skip] = true unless %w[/login /auth/callback /logout].include?(request.path_info)
-    end
-
     get "/login" do
       session[:oauth_state] = SecureRandom.hex(16)
       redirect OAUTH_CLIENT.auth_code.authorize_url(