Add AI-powered issue complexity badge (frontend-only)

[Harini-2811]

Description

Adds an AI-powered Issue Complexity Badge to the Debug results view. Each detected issue
is automatically scored (Easy 🟢 / Medium 🟡 / Hard 🔴) using a lightweight frontend
heuristic based on data already returned by the existing debug API: severity
(error/warning/info), description length, presence of complexity-related keywords
(e.g. "race condition", "memory leak", "validation"), and whether a fix suggestion was
provided. The badge appears inline next to each issue with a hover tooltip explaining
the score breakdown. Implemented entirely in the frontend (index.html, plus mirrored in
script.js/style.css) — no backend, schema, or API changes, so existing workflows and CI
checks are unaffected.

Related Issue

Fixes #1111

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features (Level 2 and 3 issues)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Screenshots

Before:

After:

Test evidence

pytest -v
# backend test suite is unaffected since no backend files were changed —
# paste your actual local pytest output here if you ran it