fix(dashboard): add custom error UI and fallback SVG for missing users

[krishnamanocha21]

Description

Fixes #64

This PR implements a custom, CommitPulse-themed Error UI and fallback SVG for scenarios where a searched GitHub username does not exist. It ensures that both the dashboard page and the raw API endpoint handle 404s gracefully without crashing or rendering generic server errors.

Key Changes:

1. Frontend (app/(root)/[username]/error.tsx):

   • Replaced the generic error boundary with a sleek "Broken Monolith" (🪨🔨) design.
   • Utilized useParams to dynamically display the missing username in the error message (e.g., User "username" not found on GitHub).
   • Hid the "Try again" button specifically for 404 errors, ensuring the user is directed to the "Go back home" CTA instead.

2. Backend / API (app/api/streak/route.ts):

   • Updated the error catch block to check for "not found" messages and return a proper 404 status code.
   • Replaced the generic 500 error SVG with a premium 600x420 "Broken Monolith" SVG that matches the project's high-fidelity aesthetic, complete with a faded/broken grid and "USER_NOT_FOUND" status text.

Pillar

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

Visual Preview

API SVG Error Fallback:

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).
• I have run npm run format and npm run lint locally and resolved all errors (CI will fail otherwise).
• 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.
• I have started the repo.
• I have made sure that i have only one commit to merge in this PR.
• The SVG output matches the CommitPulse "premium quality" aesthetic standard (no raw elements, smooth animations, correct fonts).

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