fix: keyboard trap for GitHub modal and tooltip aria-hidden

[EshaniKatiyar]

Summary

Fixes two keyboard accessibility issues flagged in #25.

Changes

templates/index.html

• Added aria-hidden="true" to decorative tooltips so screen readers skip them

static/script.js

• Implemented focus trap for the GitHub modal — Tab and Shift+Tab now cycle within the modal while it's open

• Focus returns to the trigger element on modal close

Testing

• Tabbed through the page with no mouse — focus no longer escapes the modal

• No visual changes

Closes #25

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[EshaniKatiyar]

@komalharshita Pls review this

[komalharshita]

Looks good to merge!