fix: prevent current streak overstatement when today's data is missing (JhaSourav07#2062)

## Description

Fixes JhaSourav07#2053 

This PR fixes an edge case in streak calculation that could overstate
the current streak when today's date is missing from the returned
calendar data.

### Changes

* Updated `findTodayIndex()` to return `-1` when today's date is not
present in the calendar.
* Modified `calculateStreak()` to fall back to the last available day
**only when the local date is strictly after the calendar's last date**
(indicating stale data).
* Removed the previous behavior that always used the last available day
whenever today could not be found.

### Why

Previously, when today's date was missing from the returned calendar,
the streak calculation would automatically use the last available day.
This could incorrectly report an active streak for partial or stale
calendar responses.

The new logic ensures that:

* Missing today's date does not automatically imply an active streak.
* Fallback behavior is only used for genuinely stale calendar data.
* Current streak values are not overstated.
**Modified file:** `calculate.ts`

## Pillar

- [ ] 🎨 Pillar 1 — New Theme Design
- [ ] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [x] 🛠️ Other (Bug fix, refactoring, docs)

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
- [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
- [ ] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have started the repo.
- [x] I have made sure that i have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/calculate.ts

@@ -19,7 +19,11 @@ export function findTodayIndex(days: ContributionDay[], timezone: string, now: D
 
   const localTodayIndex = days.findIndex((d) => d.date === localTodayStr);
 
-  return localTodayIndex !== -1 ? localTodayIndex : days.length - 1;
+  // If today's date isn't present in the calendar, return -1 so callers can
+  // decide whether falling back to the last available day is appropriate.
+  // Previously we always returned the last index which could cause an
+  // overstated current streak when the calendar is partial or stale.
+  return localTodayIndex !== -1 ? localTodayIndex : -1;
 }
 
 export function calculateStreak(
@@ -47,15 +51,40 @@ export function calculateStreak(
 
   // 2. Calculate Current Streak (Backwards loop with Grace Period)
   const localTodayStr = new Intl.DateTimeFormat('en-CA', { timeZone: timezone }).format(now);
-  const todayIndex = findTodayIndex(days, timezone, now);
+  let todayIndex = findTodayIndex(days, timezone, now);
 
+  // If the calendar doesn't contain today's date, only fall back to the
+  // last available day when the local date is after the calendar's last
+  // reported date (i.e. the calendar is stale). Otherwise, avoid guessing
+  // and treat today's data as missing to prevent overstating the streak.
   if (todayIndex < 0) {
-    return {
-      currentStreak: 0,
-      longestStreak: 0,
-      totalContributions: calendar.totalContributions,
-      todayDate: localTodayStr,
-    };
+    const lastIndex = days.length - 1;
+    if (lastIndex < 0) {
+      return {
+        currentStreak: 0,
+        longestStreak: 0,
+        totalContributions: calendar.totalContributions,
+        todayDate: localTodayStr,
+      };
+    }
+
+    const lastDateStr = days[lastIndex].date;
+
+    // Compare YYYY-MM-DD strings lexicographically — this works for ISO dates.
+    if (localTodayStr > lastDateStr) {
+      // Local date is after the last reported date → calendar is stale.
+      todayIndex = lastIndex;
+    } else {
+      // Calendar contains dates after (or unrelated to) local today, or
+      // today is simply missing from a partial range — don't assume the
+      // streak is alive based on the last day.
+      return {
+        currentStreak: 0,
+        longestStreak,
+        totalContributions: calendar.totalContributions,
+        todayDate: localTodayStr,
+      };
+    }
   }
 
   let isStreakAlive = false;