refactor: shared components, ConfirmDialog migration, pinned flow fixes

- Create shared Spinner component with sm/md/lg sizes
- Migrate 13 page-level spinners to shared Spinner
- Promote EmptyState to shared component, adopt in MyShares and SessionHistory
- Replace window.confirm with ConfirmDialog in 3 files
- Fix PinnedFlow.tree_type to include maintenance, update emoji display
- Verify sidebar unpin handler already correct (no-op)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/SessionHistoryPage.tsx

@@ -6,6 +6,8 @@ import type { Session, TreeListItem } from '@/types'
 import type { DateRange } from 'react-day-picker'
 import { SessionFilters } from '@/components/session/SessionFilters'
 import type { SessionFilterState } from '@/components/session/SessionFilters'
+import { Spinner } from '@/components/common/Spinner'
+import { EmptyState } from '@/components/common/EmptyState'
 import { cn } from '@/lib/utils'
 import { toast } from '@/lib/toast'
 import { getSessionResumePath } from '@/lib/routing'
@@ -188,27 +190,22 @@ export function SessionHistoryPage() {
       {/* Loading State */}
       {isLoading ? (
         <div className="flex justify-center py-12">
-          <div className="h-8 w-8 animate-spin rounded-full border-4 border-border border-t-primary" />
+          <Spinner />
         </div>
       ) : sessions.length === 0 ? (
-        <div className="py-12 text-center text-muted-foreground">
-          No sessions found.{' '}
-          {filters.ticketNumber || filters.clientName || filters.treeName || filters.dateRange?.from ? (
-            <button
-              onClick={handleClearFilters}
-              className="text-foreground hover:underline"
-            >
-              Clear filters
-            </button>
-          ) : (
-            <button
-              onClick={() => navigate('/trees')}
-              className="text-foreground hover:underline"
-            >
-              Start a new session
-            </button>
-          )}
-        </div>
+        <EmptyState
+          title="No sessions found"
+          description={filters.ticketNumber || filters.clientName || filters.treeName || filters.dateRange?.from
+            ? "Try adjusting your filters"
+            : "Complete a flow to see it here"}
+          action={
+            (filters.ticketNumber || filters.clientName || filters.treeName || filters.dateRange?.from) ? (
+              <button onClick={handleClearFilters} className="text-foreground hover:underline text-sm">
+                Clear all filters
+              </button>
+            ) : undefined
+          }
+        />
       ) : (
         <div className="space-y-4">
           {sessions.map((session) => (