feat(themes): add random theme selection via ?theme=random (JhaSourav07#115)

## Description

Fixes JhaSourav07#66

Adds support for ?theme=random as a valid theme value on the /api/streak
endpoint. When passed, the API dynamically picks a random theme from the
available themes object on every request, giving users a surprise theme
on every load.
- Changes made in app/api/streak/route.ts:
Added an isRandomTheme flag that checks if theme === 'random'. If true,
a random key is picked from the themes object using Math.random(), with
a || themes.dark fallback to ensure no crash if the selection somehow
fails.

- Also extended the cacheControl condition to disable caching when
isRandomTheme is true:
const cacheControl = refresh || isRandomTheme
  ? 'no-cache, no-store, must-revalidate'
: `public, s-maxage=${secondsToMidnight}, stale-while-revalidate=86400`;

Without this, the CDN would cache the first random SVG and serve the
same theme to all users for hours, defeating the purpose of the feature
entirely.

- Changes made in README.md:
Added random to the Theme Presets table with _varies_ for all color
columns since the values change dynamically on every load.

No other files were modified.

## Pillar

- 🛠️ Other (Bug fix, refactoring, docs)

## Working Video

https://collection.cloudinary.com/dqvm8dce2/73286b27f012f3021856fee1142847bf

## Checklist before requesting a review:

- I have read the `CONTRIBUTING.md` file.
- I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME&theme=random`).
- I have run `npm run format` and `npm run lint` locally and resolved
all errors.
- My commits follow the Conventional Commits format.
- I have updated `README.md` added a new 'random' theme preset.
- I have started the repo.
- I have made sure that i have only one commit to merge in this PR.

Author: JhaSourav07

README.md

@@ -94,14 +94,15 @@ URL Parameter > Theme Default > System Fallback
 
 ### Theme Presets
 
-| Theme              | Preview             | `bg`     | `accent` | `text`   |
-| ------------------ | ------------------- | -------- | -------- | -------- |
-| `auto`             | System light / dark | _adapts_ | _adapts_ | _adapts_ |
-| `dark` _(default)_ | GitHub dark         | `0d1117` | `58a6ff` | `c9d1d9` |
-| `neon`             | Cyberpunk           | `000000` | `ff00ff` | `00ffcc` |
-| `dracula`          | Dracula Pro         | `282a36` | `bd93f9` | `f8f8f2` |
-| `github`           | GitHub green        | `0d1117` | `238636` | `ffffff` |
-| `light`            | Clean & minimal     | `ffffff` | `0969da` | `24292f` |
+| Theme              | Preview                  | `bg`     | `accent` | `text`   |
+| ------------------ | ------------------------ | -------- | -------- | -------- |
+| `auto`             | System light / dark      | _adapts_ | _adapts_ | _adapts_ |
+| `dark` _(default)_ | GitHub dark              | `0d1117` | `58a6ff` | `c9d1d9` |
+| `neon`             | Cyberpunk                | `000000` | `ff00ff` | `00ffcc` |
+| `dracula`          | Dracula Pro              | `282a36` | `bd93f9` | `f8f8f2` |
+| `github`           | GitHub green             | `0d1117` | `238636` | `ffffff` |
+| `light`            | Clean & minimal          | `ffffff` | `0969da` | `24292f` |
+| `random`           | Surprise theme on reload | _varies_ | _varies_ | _varies_ |
 
 > **`auto` uses CSS `@media (prefers-color-scheme)`** inside the SVG so the badge switches between the `light` and `dark` palettes based on the viewer's OS setting — no JavaScript required. This is ideal for GitHub profile READMEs where visitors may use either mode.
 

app/api/streak/route.ts

@@ -22,7 +22,16 @@ export async function GET(request: Request) {
 
     const themeName = searchParams.get('theme') || 'dark';
     const isAutoTheme = themeName === 'auto';
-    const selectedTheme = isAutoTheme ? themes.light : themes[themeName] || themes.dark;
+    const isRandomTheme = themeName === 'random';
+    const selectedTheme = (() => {
+      if (isAutoTheme) return themes.light;
+      if (isRandomTheme) {
+        const themeKeys = Object.keys(themes);
+        const randomKey = themeKeys[Math.floor(Math.random() * themeKeys.length)];
+        return themes[randomKey] || themes.dark;
+      }
+      return themes[themeName] || themes.dark;
+    })();
 
     const rawSpeed = searchParams.get('speed') || '8s';
     const speed = /^\d+(\.\d+)?s$/.test(rawSpeed) ? rawSpeed : '8s';
@@ -58,9 +67,10 @@ export async function GET(request: Request) {
 
     // 4. Calculate Cache Control (Reset at UTC Midnight)
     const secondsToMidnight = getSecondsUntilUTCMidnight();
-    const cacheControl = refresh
-      ? 'no-cache, no-store, must-revalidate'
-      : `public, s-maxage=${secondsToMidnight}, stale-while-revalidate=86400`;
+    const cacheControl =
+      refresh || isRandomTheme
+        ? 'no-cache, no-store, must-revalidate'
+        : `public, s-maxage=${secondsToMidnight}, stale-while-revalidate=86400`;
 
     // 5. Return the Image Response
     return new NextResponse(svg, {