feat(web): replace machine tree level with filter chips in session list

Single-machine users no longer expand a redundant machine layer; with
multiple machines a chip filter bar (persisted, with hover health popup)
replaces the collapsible machine headers. Directory groups now render
top-level with machine-name suffixes when unfiltered. Also removes the
redundant session/project count header text.

web/src/hooks/useSessionListMachineFilter.test.ts

@@ -0,0 +1,28 @@
+import { beforeEach, describe, expect, it } from 'vitest'
+import {
+    DEFAULT_SESSION_LIST_MACHINE_FILTER,
+    getInitialSessionListMachineFilter,
+} from './useSessionListMachineFilter'
+
+describe('useSessionListMachineFilter helpers', () => {
+    beforeEach(() => {
+        window.localStorage.clear()
+    })
+
+    it('defaults to null (all machines) for missing or blank storage values', () => {
+        expect(getInitialSessionListMachineFilter()).toBe(DEFAULT_SESSION_LIST_MACHINE_FILTER)
+        expect(getInitialSessionListMachineFilter()).toBeNull()
+
+        window.localStorage.setItem('hapi-session-list-machine-filter', '')
+        expect(getInitialSessionListMachineFilter()).toBeNull()
+
+        window.localStorage.setItem('hapi-session-list-machine-filter', '   ')
+        expect(getInitialSessionListMachineFilter()).toBeNull()
+    })
+
+    it('reads a stored machine id', () => {
+        window.localStorage.setItem('hapi-session-list-machine-filter', 'machine-1')
+
+        expect(getInitialSessionListMachineFilter()).toBe('machine-1')
+    })
+})

web/src/hooks/useSessionListMachineFilter.ts

@@ -0,0 +1,91 @@
+import { useCallback, useEffect, useState } from 'react'
+
+// null = "All machines" (no filtering). A string is a machine id, or
+// UNKNOWN_MACHINE_ID ('__unknown__') for sessions without machine metadata.
+export type SessionListMachineFilter = string | null
+
+export const DEFAULT_SESSION_LIST_MACHINE_FILTER: SessionListMachineFilter = null
+
+function getSessionListMachineFilterStorageKey(): string {
+    return 'hapi-session-list-machine-filter'
+}
+
+function isBrowser(): boolean {
+    return typeof window !== 'undefined' && typeof document !== 'undefined'
+}
+
+function safeGetItem(key: string): string | null {
+    if (!isBrowser()) {
+        return null
+    }
+    try {
+        return localStorage.getItem(key)
+    } catch {
+        return null
+    }
+}
+
+function safeSetItem(key: string, value: string): void {
+    if (!isBrowser()) {
+        return
+    }
+    try {
+        localStorage.setItem(key, value)
+    } catch {
+        // Ignore storage errors
+    }
+}
+
+function safeRemoveItem(key: string): void {
+    if (!isBrowser()) {
+        return
+    }
+    try {
+        localStorage.removeItem(key)
+    } catch {
+        // Ignore storage errors
+    }
+}
+
+function parseSessionListMachineFilter(raw: string | null): SessionListMachineFilter {
+    return raw && raw.trim().length > 0 ? raw : DEFAULT_SESSION_LIST_MACHINE_FILTER
+}
+
+export function getInitialSessionListMachineFilter(): SessionListMachineFilter {
+    return parseSessionListMachineFilter(safeGetItem(getSessionListMachineFilterStorageKey()))
+}
+
+export function useSessionListMachineFilter(): {
+    machineFilter: SessionListMachineFilter
+    setMachineFilter: (filter: SessionListMachineFilter) => void
+} {
+    const [machineFilter, setMachineFilterState] = useState<SessionListMachineFilter>(getInitialSessionListMachineFilter)
+
+    useEffect(() => {
+        if (!isBrowser()) {
+            return
+        }
+
+        const onStorage = (event: StorageEvent) => {
+            if (event.key !== getSessionListMachineFilterStorageKey()) {
+                return
+            }
+            setMachineFilterState(parseSessionListMachineFilter(event.newValue))
+        }
+
+        window.addEventListener('storage', onStorage)
+        return () => window.removeEventListener('storage', onStorage)
+    }, [])
+
+    const setMachineFilter = useCallback((filter: SessionListMachineFilter) => {
+        setMachineFilterState(filter)
+
+        if (filter === null) {
+            safeRemoveItem(getSessionListMachineFilterStorageKey())
+        } else {
+            safeSetItem(getSessionListMachineFilterStorageKey(), filter)
+        }
+    }, [])
+
+    return { machineFilter, setMachineFilter }
+}