feat: add todo tracking and custom tool views

Adds todo tracking infrastructure with TodoWrite tool integration to extract
and persist todos in sessions. Refactors ToolCard component to support
custom tool view rendering and displays todo progress in SessionList.

web/src/components/ToolCard/views/MultiEditView.tsx

@@ -0,0 +1,37 @@
+import type { ToolViewProps } from '@/components/ToolCard/views/_all'
+import { DiffView } from '@/components/DiffView'
+
+function isObject(value: unknown): value is Record<string, unknown> {
+    return Boolean(value) && typeof value === 'object'
+}
+
+type Edit = { old_string: string; new_string: string }
+
+function extractEdits(input: unknown): Edit[] {
+    if (!isObject(input) || !Array.isArray(input.edits)) return []
+    return input.edits
+        .filter(isObject)
+        .map((edit) => ({
+            old_string: typeof edit.old_string === 'string' ? edit.old_string : '',
+            new_string: typeof edit.new_string === 'string' ? edit.new_string : ''
+        }))
+        .filter((edit) => edit.old_string.length > 0 || edit.new_string.length > 0)
+}
+
+export function MultiEditView(props: ToolViewProps) {
+    const edits = extractEdits(props.block.tool.input)
+    if (edits.length === 0) return null
+
+    return (
+        <div className="flex flex-col gap-2">
+            {edits.map((edit, idx) => (
+                <DiffView
+                    key={idx}
+                    oldString={edit.old_string}
+                    newString={edit.new_string}
+                />
+            ))}
+        </div>
+    )
+}
+