fix: a new view opens at its top instead of the last view's scroll offset

The router had no scrollBehavior, so Settings entered from a scrolled feed
opened scrolled too, its heading and tab strip under the nav. Back/forward
restores the saved position, and a new query on the same view keeps its
place.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

frontend/test/router.spec.js

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest'
-import router, { FRONT_DOOR } from '../src/router.js'
+import router, { FRONT_DOOR, scrollBehavior } from '../src/router.js'
 
 describe('router', () => {
   it('FRONT_DOOR is the post feed', () => {
@@ -74,4 +74,26 @@ describe('router', () => {
     expect(r.name).toBe('series-read')
     expect(r.meta.immersive).toBe(true)
   })
+
+  describe('scrollBehavior', () => {
+    const at = (path) => ({ path, hash: '', matched: [{}] })
+
+    it('opens a new view at its top, not at the last view\'s offset', () => {
+      expect(scrollBehavior(at('/settings'), at('/latest'), null)).toEqual({ top: 0 })
+    })
+
+    it('back and forward return to where the entry was left', () => {
+      const saved = { left: 0, top: 1234 }
+      expect(scrollBehavior(at('/latest'), at('/settings'), saved)).toBe(saved)
+    })
+
+    it('a new query on the same view keeps its place', () => {
+      expect(scrollBehavior(at('/gallery'), at('/gallery'), null)).toBe(false)
+    })
+
+    it('the first navigation of a page load opens at the top', () => {
+      const initial = { path: '/', hash: '', matched: [] }
+      expect(scrollBehavior(at('/latest'), initial, null)).toEqual({ top: 0 })
+    })
+  })
 })