fix(participants): show Me contextually per logged-in user

Participant id=1 was named "Me" in the DB, causing Sonu and other users
to see "Me" referring to Siddharth when viewing splits and shared expenses.

- Rename participant id=1 from "Me" to "Siddharth" in the DB
- /api/participants now substitutes "Me" for whichever participant matches
  the current user, so the label is always relative to the viewer
- split-modal: default split uses currentUser.id instead of name === "Me"
- transactions/page: filter and display logic uses participant ID not name
- shared/page: split chips show "Me" when participant_id === current user

Also includes add-transaction-modal tags support (pre-existing staged change).

src/components/split-modal.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useState, useEffect } from "react";
-import { useParticipants, useSetSplits, useTransactionSplits, useBulkAction, useCreateRule } from "@/lib/hooks";
+import { useParticipants, useSetSplits, useTransactionSplits, useBulkAction, useCreateRule, useCurrentUser } from "@/lib/hooks";
 
 interface Split {
   participant_id: number;
@@ -22,6 +22,7 @@ export function SplitModal({ transactionId, transactionIds, amount, description,
   const singleId = transactionId ?? 0;
 
   const { data: participants } = useParticipants();
+  const { data: currentUser } = useCurrentUser();
   const { data: existingSplits } = useTransactionSplits(isBulk ? 0 : singleId);
   const setSplits = useSetSplits();
   const bulkAction = useBulkAction();
@@ -34,10 +35,9 @@ export function SplitModal({ transactionId, transactionIds, amount, description,
 
   // Initialise: bulk always defaults to 100% Me; single loads existing splits
   useEffect(() => {
-    if (!participants || participants.length === 0) return;
-    const me = participants.find((p) => p.name === "Me");
+    if (!participants || participants.length === 0 || !currentUser) return;
     if (isBulk) {
-      if (me) setSplitsState([{ participant_id: me.id, share_percent: 100 }]);
+      setSplitsState([{ participant_id: currentUser.id, share_percent: 100 }]);
     } else if (existingSplits && existingSplits.length > 0) {
       setSplitsState(
         existingSplits.map((s: { participant_id: number; share_percent: number }) => ({
@@ -45,10 +45,10 @@ export function SplitModal({ transactionId, transactionIds, amount, description,
           share_percent: Number(s.share_percent),
         }))
       );
-    } else if (me) {
-      setSplitsState([{ participant_id: me.id, share_percent: 100 }]);
+    } else {
+      setSplitsState([{ participant_id: currentUser.id, share_percent: 100 }]);
     }
-  }, [existingSplits, participants, isBulk]);
+  }, [existingSplits, participants, isBulk, currentUser]);
 
   const total = splits.reduce((sum, s) => sum + s.share_percent, 0);