fix: reduce page transition lag on Android (closes #48)

Two causes of ~1s navigation delay fixed:

1. glass.css Section 19 was extending the page-in animation from 0.20s
   to 0.30s using spring easing. Reverted to 0.20s in / 0.12s out.

2. During transitions, dozens of backdrop-filter layers (widgets, cards,
   inputs, toolbars) were composited simultaneously for both the outgoing
   and incoming page, overloading mid-range Android GPUs.
   Added html.navigating class: router.js sets it at transition start,
   glass.css overrides all app-content backdrop-filters to none while
   active, animationend removes it once the in-animation completes.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "oikos",
-  "version": "0.20.5",
+  "version": "0.20.6",
   "description": "Self-hosted family planner - calendar, tasks, shopping, meal planning, budget and more. Private, open-source, no subscription.",
   "main": "server/index.js",
   "type": "module",