Make `ThreadState.filters` type more specific

This unfortunately doesn't catch mistakes where incorrect property names are
used because TS allows excess properties, but it does at least document what the
expected keys are.

src/sidebar/helpers/thread-annotations.ts

@@ -4,6 +4,7 @@ import { buildThread } from './build-thread';
 import type { Thread, BuildThreadOptions } from './build-thread';
 import { filterAnnotations } from './filter-annotations';
 import { parseFilterQuery } from './query-parser';
+import type { FilterField } from './query-parser';
 import { shouldShowInTab } from './tabs';
 import { sorters } from './thread-sorters';
 
@@ -13,7 +14,7 @@ export type ThreadState = {
   selection: {
     expanded: Record<string, boolean>;
     filterQuery: string | null;
-    filters: Record<string, string>;
+    filters: Partial<Record<FilterField, string>>;
     forcedVisible: string[];
     selected: string[];
     sortKey: keyof typeof sorters;