feat(domain): allocation rules become a dated series

Post.standingRule becomes Post.rules: RuleVersion[], resolved by ruleAt() as
the last version effective at or before a month. Per-month overrides still win
outright; a post with no applicable version allocates zero rather than
applying its earliest rule backwards.

allocationFor keeps its signature, so the fold, the views and the ODS export
are untouched. Settings gets a read-only rule summary for now; the history
editor follows.

src/ui/routes/settings/PostsSection.tsx

@@ -4,7 +4,9 @@ import { Input } from "@/components/ui/input";
 import { useDataset } from "../../hooks/useDataset.ts";
 import { useMutate } from "../../hooks/useMutate.ts";
 import { addPost, movePost, setPostArchived, updatePost } from "../../../store/actions.ts";
-import { CURRENCIES, type Currency, type Rule } from "../../../domain/types.ts";
+import { CURRENCIES, type Currency, type Post } from "../../../domain/types.ts";
+import { ruleAt } from "../../../domain/allocation.ts";
+import { currentMonth } from "../../../store/index.ts";
 
 export function PostsSection() {
   const dataset = useDataset();
@@ -13,48 +15,19 @@ export function PostsSection() {
   const ordered = [...dataset.posts].sort((a, b) => a.order - b.order);
   const base = dataset.settings.baseCurrency;
 
-  function ruleEditor(postId: string, rule: Rule) {
+  function ruleSummary(post: Post) {
+    const effective = ruleAt(post, currentMonth);
+    if (!effective) {
+      return <span className="text-xs text-muted-foreground">not budgeted</span>;
+    }
+    const { rule } = effective;
     return (
-      <div className="flex items-center gap-2">
-        <select
-          className="h-8 rounded border bg-background px-1 text-xs"
-          value={rule.kind}
-          onChange={(event) => {
-            const kind = event.target.value;
-            mutate((draft) =>
-              updatePost(draft, postId, {
-                standingRule:
-                  kind === "fixed"
-                    ? { kind: "fixed", amount: { amount: 0, currency: base } }
-                    : { kind: "percentOfIncome", percent: 0 },
-              }),
-            );
-          }}
-        >
-          <option value="fixed">fixed amount</option>
-          <option value="percentOfIncome">% of income</option>
-        </select>
-        <Input
-          type="number"
-          step="0.01"
-          className="font-money h-8 w-28"
-          value={rule.kind === "fixed" ? rule.amount.amount : rule.percent}
-          onChange={(event) => {
-            const value = Number(event.target.value) || 0;
-            mutate((draft) =>
-              updatePost(draft, postId, {
-                standingRule:
-                  rule.kind === "fixed"
-                    ? { kind: "fixed", amount: { ...rule.amount, amount: value } }
-                    : { kind: "percentOfIncome", percent: value },
-              }),
-            );
-          }}
-        />
-        <span className="w-10 text-xs text-muted-foreground">
-          {rule.kind === "fixed" ? rule.amount.currency : "%"}
-        </span>
-      </div>
+      <span className="text-xs">
+        {rule.kind === "fixed"
+          ? `${rule.amount.amount} ${rule.amount.currency}`
+          : `${rule.percent}% of income`}
+        <span className="ml-1 text-muted-foreground">from {effective.from}</span>
+      </span>
     );
   }
 
@@ -105,7 +78,7 @@ export function PostsSection() {
                   ))}
                 </select>
               </td>
-              <td className="py-2">{ruleEditor(post.id, post.standingRule)}</td>
+              <td className="py-2">{ruleSummary(post)}</td>
               <td className="py-2 text-right">
                 <Button size="sm" variant="ghost" disabled={index === 0}
                   onClick={() => mutate((draft) => movePost(draft, post.id, -1))}>
@@ -140,12 +113,7 @@ export function PostsSection() {
           variant="outline"
           disabled={newName.trim() === ""}
           onClick={() => {
-            mutate((draft) =>
-              addPost(draft, newName.trim(), base, {
-                kind: "fixed",
-                amount: { amount: 0, currency: base },
-              }),
-            );
+            mutate((draft) => addPost(draft, newName.trim(), base));
             setNewName("");
           }}
         >