Add Spanish localization support

[Jorel97]

fixes #9

Summary

• extract user-facing UI copy into a small English/Spanish dictionary
• detect Spanish browser locales on first load and persist manual language overrides in localStorage under taskforge.locale
• add a compact header language selector and translate auth, task form, empty-state, document title, and delete aria labels
• document localization support and add focused locale tests

Validation

• npm test
• npm run lint
• node --check src/app.js
• node --check src/i18n.mjs
• node --check test/i18n.test.mjs
• npx prettier --check index.html styles.css src/app.js src/i18n.mjs test/i18n.test.mjs README.md
• git diff --check

[codebounty-dev-bot]

⚠️ Attention Sponsor: This PR was submitted by @Jorel97 who has not applied for this bounty (#9). Please verify if you want to consider this contribution.

👋 Hey @Jorel97: This issue has a bounty on CodeBounty! To be eligible for the reward, please sign up on CodeBounty and apply for this bounty. If you apply, your PR will be automatically associated with your application.

[Jorel97]

Hi CodeBounty/sponsor team, I logged in to CodeBounty as @Jorel97 and checked the repository issue list for #9. The issue currently appears with an Add Bounty action rather than an Apply button, and the GitHub issue does not show a public bounty announcement/comment that I can use to apply.

This PR was submitted after the CodeBounty bot said #9 has a bounty. Could you please confirm whether a developer application is currently possible for #9, and whether this PR can be associated if/when the bounty is available?