fix(leaderboard): make weekly period non-empty on Sundays

[evan188199-tech]

What

The weekly leaderboard (top users + your position) returns no results every Sunday. This is a bug I found while working on the leaderboard — it's not covered by #217 (which only filters by `endedAt`).

Root cause

`getDateRangeForPeriod("weekly")` in `src/features/leaderboard/lib/utils.ts` computes Monday as:

```ts
const startOfWeek = now.startOf("week").add(1, "day"); // dayjs week starts on Sunday, add 1 for Monday
```

dayjs' default week starts on Sunday. On a Sunday, `startOf("week")` is today (Sunday 00:00), so `add(1, "day")` lands on the following Monday — in the future. The range becomes `startDate > endDate`, and both leaderboard actions feed it into an inverted Prisma filter `startedAt: { gte, lte }` that matches zero sessions.

Quick table of the Monday offset (in days from today) for each weekday:

Weekday	OLD offset	NEW offset
Sunday	+1 (inverted!)	−6
Monday	0	0
Tuesday	−1	−1
…	…	…
Saturday	−5	−5

Only Sunday is wrong, but it breaks the entire weekly view one day a week.

Fix

Derive Monday from the day-of-week so it always points backwards:

```ts
const daysSinceMonday = (now.day() + 6) % 7; // Sun→6, Mon→0, …, Sat→5
const startOfWeek = now.subtract(daysSinceMonday, "day").startOf("day");
```

`monthly` and `all-time` are unaffected.

Fixes #220.

[vercel]

Someone is attempting to deploy a commit to the Workoutcool Team Team on Vercel.

A member of the Team first needs to authorize it.

[evan188199-tech]

此 PR 由 GLM-5.2 修复。