chore(ci): clear the lint debt and make lint blocking (#79)

CI ran lint with continue-on-error because 23 errors predated it. All are
fixed, so the flag is gone and a typecheck step is added beside it — nothing
else in CI compiles the test files, so a type error in a fixture could only
ever be found on a developer's machine.

Real fixes, no blanket disables and no suppressions:

- react-hooks/set-state-in-effect (6): each was "reset or seed state when an
  input changes", so each becomes a state adjustment during render behind a
  previous-value guard — the documented React pattern for it. Two were also
  latent bugs. transactions/page.tsx depended on `others`, rebuilt by .filter()
  every render, so the effect already ran every render, one commit late.
  shared/page.tsx depended on the `tagIds` array identity, so the row window
  was reset more often than "changing any filter" claims — including after
  "Show more", the one interaction it must not undo; the key now joins by
  value. csv-import-modal reads localStorage once at mount, which is a lazy
  useState initializer, not an effect (SSR-guarded, though the modal is
  mounted from client state and never prerenders).
- react-hooks/static-components (3): SortIcon was declared inside SharedPage,
  making it a new component type on every render. Hoisted to module scope with
  the sort state passed as props.
- @typescript-eslint/no-explicit-any (9): the dynamically imported route
  handlers in two integration tests. Typed as their real signatures with one
  `as unknown as NextRequest` cast at the fixture boundary, so the call sites
  and response bodies get checked instead of opted out.
- react/no-unescaped-entities (5): exact-equivalent entities (', "),
  so no rendered text changes.

Also fixes the one pre-existing tsc error (unit/order-ingestion.test.ts:511):
`options` is optional, and `?? 0` keeps the assertion failing on an absent
list rather than silencing it with a non-null bang.

9 warnings remain (no-unused-vars, exhaustive-deps). eslint exits 0 on
warnings, so lint blocks on errors only — deliberately, since none of the
remaining warnings should stop a push.

src/components/split-modal.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useState, useEffect } from "react";
+import { useState } from "react";
 import { useParticipants, useSetSplits, useTransactionSplits, useBulkAction, useCreateRule, useCurrentUser } from "@/lib/hooks";
 
 interface Split {
@@ -33,22 +33,32 @@ export function SplitModal({ transactionId, transactionIds, amount, description,
   const [saveAsRule, setSaveAsRule] = useState(false);
   const [ruleSaved, setRuleSaved] = useState(false);
 
-  // Initialise: bulk always defaults to 100% Me; single loads existing splits
-  useEffect(() => {
-    if (!participants || participants.length === 0 || !currentUser) return;
-    if (isBulk) {
-      setSplitsState([{ participant_id: currentUser.id, share_percent: 100 }]);
-    } else if (existingSplits && existingSplits.length > 0) {
-      setSplitsState(
-        existingSplits.map((s: { participant_id: number; share_percent: number }) => ({
-          participant_id: s.participant_id,
-          share_percent: Number(s.share_percent),
-        }))
-      );
-    } else {
-      setSplitsState([{ participant_id: currentUser.id, share_percent: 100 }]);
-    }
-  }, [existingSplits, participants, isBulk, currentUser]);
+  // Initialise: bulk always defaults to 100% Me; single loads existing splits.
+  //
+  // Seeded during render behind a previous-value guard rather than in an
+  // effect. Both queries resolve after the first paint, so as an effect this
+  // rendered an empty split table and then replaced it — and, worse, it re-ran
+  // on every identity change of `participants`/`existingSplits`, which would
+  // discard edits in progress if either query refetched while the modal was
+  // open. Keying on the inputs that actually define the seed fixes both.
+  const seedKey =
+    !participants || participants.length === 0 || !currentUser
+      ? null
+      : `${isBulk ? "bulk" : singleId}:${currentUser.id}:${
+          existingSplits ? JSON.stringify(existingSplits) : "none"
+        }`;
+  const [seededFrom, setSeededFrom] = useState<string | null>(null);
+  if (seedKey !== null && seedKey !== seededFrom) {
+    setSeededFrom(seedKey);
+    setSplitsState(
+      !isBulk && existingSplits && existingSplits.length > 0
+        ? existingSplits.map((s: { participant_id: number; share_percent: number }) => ({
+            participant_id: s.participant_id,
+            share_percent: Number(s.share_percent),
+          }))
+        : [{ participant_id: currentUser!.id, share_percent: 100 }]
+    );
+  }
 
   const total = splits.reduce((sum, s) => sum + s.share_percent, 0);