test: cover drag upload language sync

[saurabhhhcodes]

Summary

• add Playwright regression coverage for drag-and-drop uploads auto-selecting the detected language tab
• update the static sample validator to run under the repo's ESM test package
• correct the Python sample header so the existing sample-comment validator passes

Why

Issue #308 reports that dropped files could leave the active language tab stale. Current main already routes both file picker and drag/drop through handleUploadedFile, so this PR protects that behavior with a focused regression test and fixes the existing frontend static validator path.

Validation

• npm run test:static passes from frontend/
• git diff --check passes

Local limitation

• Targeted Playwright run was attempted, but local execution is blocked by workstation backend setup: system python is absent, Python 3.9 cannot import the app because it needs 3.10+ union typing, and uv with Python 3.11 then stops on missing native libmagic. The GitHub runner has the project dependency stack and should be the source of truth for the E2E workflow.

Closes #308

[saurabhhhcodes]

Pushed a static-check repair follow-up after the first run failed in frontend/index.html validation.

What changed in the follow-up:

• removed invalid label role/aria-label usage around the hidden file input
• fixed result tab/tabpanel associations
• changed language selector buttons to pressed-state controls instead of incomplete tabs
• corrected heading order in the hero pills
• replaced the spinner child div inside the analyze button with a span
• normalized the Hindi sample text and removed validator-unfriendly void-element trailing slashes

Validation now passing locally:

• npx --yes w3c-html-validator frontend/index.html
• npm run test:static
• git diff --check

[saurabhhhcodes]

All checks are green now:

• frontend static checks
• Frontend Check
• Lint
• test (3.11)
• test (3.12)
• analyze

This PR closes assigned issue #308 and currently has no labels. If accepted for GSSoC, could you please add the relevant tracking labels such as gssoc:approved, level:beginner, type:bug, and type:frontend?

[saurabhhhcodes]

Refreshed this branch against current main and resolved the frontend/index.html conflict.

Conflict handled:

• kept the main branch upload control as a button for valid semantics
• preserved this PR’s static-check fix by keeping the file input outside the button and avoiding validator-unfriendly void-element syntax

Validation:

• npx --yes w3c-html-validator frontend/index.html (pass)
• npm run test:static from frontend (pass)
• git diff --check

Notes:

• full local pytest is blocked on this machine because only Python 3.9 is installed; the repo now uses 3.11+/3.12 syntax such as str | None and datetime.UTC
• Playwright webServer is also blocked locally because its config calls python, which is not installed as a command here; GitHub CI is rerunning the 3.11/3.12 and frontend checks.

[saurabhhhcodes]

Refreshed the branch against latest main again and resolved the recurring frontend/index.html conflict. I kept the upload file input outside the button and preserved the validator-safe tab/language-control fixes from this PR.\n\nValidation after the refresh:\n- npx --yes w3c-html-validator index.html from frontend/\n- npm run test:static from frontend/\n- git diff --check HEAD~1..HEAD\n\nGitHub checks are rerunning on commit c09a842.