fix: hoist window.addSkill and remove duplicate broken submit listener

[nancysangani]

Fixes #810

Three bugs in initIndexPage() in script.js caused all skill interactions to silently fail.

Root causes:

1. skillsTextInput referenced but never declared — threw ReferenceError when form was submitted, preventing recommendations from generating
2. window.addSkill defined inside the first submit handler instead of at initIndexPage() scope — chip click handlers and the Enter key handler found it undefined before any form submission occurred
3. Duplicate form.addEventListener("submit") binding — the broken first listener was never removed when the correct second one was added

Changes:

• static/script.js — hoisted window.addSkill to top of initIndexPage() scope; removed the entire broken first submit listener; no changes to the correct second submit listener
• tests/test_skills_input.py — 35 tests covering DOM landmark presence, chip data-skill attributes, skillsTextInput absence, window.addSkill hoisting, single submit listener, Enter key wiring, and API payload contract

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[nancysangani]

Hi @komalharshita, please review the PR when you get a chance. Thanks!