feat(web): add stepwise session preview controls (#1266)

* feat(web): add stepwise session preview controls

* fix(web): avoid no-op session preview collapse

* fix(web): expand previews from rendered count

web/src/components/SessionList.test.ts

@@ -6,6 +6,7 @@ import {
     filterActiveSessionsOnly,
     getSessionTimeRange,
     getNextSessionVisibleCount,
+    getPreviousSessionVisibleCount,
     getSessionDedupKey,
     getWorktreeSessionLabel,
     getVisibleSessionPreview,
@@ -428,6 +429,22 @@ describe('getNextSessionVisibleCount', () => {
     })
 })
 
+describe('getPreviousSessionVisibleCount', () => {
+    it('collapses one batch of step size per call', () => {
+        expect(getPreviousSessionVisibleCount(20, 8)).toBe(12)
+        expect(getPreviousSessionVisibleCount(12, 8)).toBe(8)
+    })
+
+    it('never goes below the preview limit', () => {
+        expect(getPreviousSessionVisibleCount(10, 8)).toBe(8)
+        expect(getPreviousSessionVisibleCount(8, 8)).toBe(8)
+    })
+
+    it('uses a minimum batch size of one', () => {
+        expect(getPreviousSessionVisibleCount(5, 0)).toBe(4)
+    })
+})
+
 describe('expandSelectedSessionCollapseOverrides', () => {
     it('expands the collapsed project group, but preserves session preview folding', () => {
         const overrides = new Map<string, boolean>([