docs(github): add JSDoc comments for fetch utility functions (JhaSourav07#772)

## Description

Added JSDoc documentation for the public GitHub fetch utility functions
in `lib/github.ts`.

### Changes made

* Added documentation for all three exported fetch functions
* Added `@param`, `@returns`, `@throws`, and `@example` tags
* Documented `FetchOptions` fields including `bypassCache`, `from`, and
`to`
* Added notes about retry behavior and possible error responses
* No functional changes

Fixes JhaSourav07#311

## Pillar

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

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src="https://github.com/user-attachments/assets/fe328081-21fd-40c1-9414-3e2dafbf4944"
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## 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): ...`).
- [x] 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.
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standard (no raw elements, smooth animations, correct fonts).
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contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/github.ts

@@ -92,10 +92,30 @@ type GitHubContributionResponse = {
   errors?: Array<{ message: string }>;
 };
 
+/**
+ * Configuration options for GitHub API fetch requests.
+ */
 type FetchOptions = {
+  /**
+   * Skips the in-memory cache and forces a fresh GitHub API request.
+   */
   bypassCache?: boolean;
+
+  /**
+   * Start date used for filtering contribution data.
+   * Expected format: YYYY-MM-DD.
+   */
   from?: string;
+
+  /**
+   * End date used for filtering contribution data.
+   * Expected format: YYYY-MM-DD.
+   */
   to?: string;
+
+  /**
+   * Optional AbortSignal used to cancel the request.
+   */
   signal?: AbortSignal;
 };
 
@@ -159,6 +179,35 @@ export function displayName(profile: GitHubUserProfile): string {
   return profile.login;
 }
 
+/**
+ * Fetches a user's GitHub contribution calendar using the GitHub GraphQL API.
+ *
+ * Requests are automatically retried on rate limiting (429) and server errors (5xx)
+ * using exponential backoff.
+ *
+ * @param username - GitHub username to fetch contributions for.
+ * @param options - Optional fetch configuration.
+ * @param options.bypassCache - Forces a fresh API request instead of using cached data.
+ * @param options.from - Start date for contribution filtering.
+ * @param options.to - End date for contribution filtering.
+ * @param options.signal - Optional AbortSignal used to cancel the request.
+ *
+ * @returns A promise resolving to the user's contribution calendar.
+ *
+ * @throws {Error} If the GitHub PAT is missing or invalid.
+ * @throws {Error} If the GitHub user cannot be found.
+ * @throws {Error} If the GitHub API request fails after all retry attempts.
+ * @throws {Error} If the request times out or is aborted.
+ *
+ * @example
+ * ```ts
+ * const calendar = await fetchGitHubContributions("octocat", {
+ *   from: "2025-01-01",
+ *   to: "2025-12-31",
+ *   bypassCache: true,
+ * });
+ * ```
+ */
 export async function fetchGitHubContributions(
   username: string,
   options: FetchOptions = {}
@@ -232,6 +281,32 @@ export async function fetchGitHubContributions(
   return calendar;
 }
 
+/**
+ * Fetches public GitHub profile information for a user.
+ *
+ * Requests are automatically retried on rate limiting (429) and server errors (5xx)
+ * using exponential backoff.
+ *
+ * @param username - GitHub username to fetch profile data for.
+ * @param options - Optional fetch configuration.
+ * @param options.bypassCache - Forces a fresh API request instead of using cached data.
+ * @param options.signal - Optional AbortSignal used to cancel the request.
+ *
+ * @returns A promise resolving to the user's GitHub profile data.
+ *
+ * @throws {Error} If the GitHub user cannot be found.
+ * @throws {Error} If the GitHub REST API request fails.
+ * @throws {Error} If the request times out or is aborted.
+ *
+ * @example
+ * ```ts
+ * const controller = new AbortController();
+ *
+ * const profile = await fetchUserProfile("octocat", {
+ *   signal: controller.signal,
+ * });
+ * ```
+ */
 export async function fetchUserProfile(
   username: string,
   options: FetchOptions = {}
@@ -269,6 +344,32 @@ export async function fetchUserProfile(
   return profile;
 }
 
+/**
+ * Fetches public repositories for a GitHub user.
+ *
+ * Repository data is fetched from the GitHub REST API with automatic retries
+ * for rate limiting (429) and server errors (5xx).
+ *
+ * Results are paginated with a maximum limit of 300 repositories
+ * across 3 API pages.
+ *
+ * @param username - GitHub username to fetch repositories for.
+ * @param options - Optional fetch configuration.
+ * @param options.bypassCache - Forces a fresh API request instead of using cached data.
+ * @param options.signal - Optional AbortSignal used to cancel the request.
+ *
+ * @returns A promise resolving to an array of GitHub repositories.
+ *
+ * @throws {Error} If the GitHub API request fails after all retry attempts.
+ * @throws {Error} If the request times out or is aborted.
+ *
+ * @example
+ * ```ts
+ * const repos = await fetchUserRepos("octocat");
+ *
+ * console.log(repos.length);
+ * ```
+ */
 export async function fetchUserRepos(
   username: string,
   options: FetchOptions = {}