[Task] Update /about page with all current features

[RMCampos]

Description

The /about page is outdated. It currently lists only 3 features while the app has grown to support ~9+ distinct capabilities. Several shipped features are not mentioned, and there is a stale reference to a "Help page (in the future)" that does not exist.

Current State

The About page (client/src/views/About/index.tsx) currently mentions:

• Quickly add and manage tasks and notes
• Search and filter notes for easy access
• Intuitive and clean user interface

Missing Features

The following shipped features are not reflected on the About page:

• Dark/light theme support — responsive design with theme toggle
• URL attachments for tasks and notes — attach links to tasks and notes
• Tagging system (#tag) — organize with tags, including multiple tags per item
• Public note sharing — share notes via public links
• Markdown formatting toolbar — rich text editing with markdown support
• Email confirmation required — account security via email verification
• Last activity tracking — visible last activity timestamp
• Internationalization (4 languages) — EN, PT, ES, RU support
• Mobile App / PWA — native mobile apps and PWA support

Stale Content

• Remove or reword the "Help page (in the future)" reference.

Technical Requirements

• Update client/src/views/About/index.tsx to list all current features
• Update translation strings in all 4 language files:
  • client/src/constants/english.ts
  • client/src/constants/portuguese.ts
  • client/src/constants/spanish.ts
  • client/src/constants/russian.ts
• Update or add test coverage in client/src/__test__/views/About.test.tsx
• Consider updating the tech stack list to include Vite, React Router 7, i18next, Spring Boot 4.x

Acceptance Criteria

• About page lists all major current features (minimum: tags, sharing, charts, dark mode, attachments, markdown, i18n)
• Stale "Help page (in the future)" reference removed or reworded
• All 4 language translations updated consistently
• Tests pass: bash tools/check-frontend.sh
• CHANGELOG.md file is updated with changes