fix(web): restore mobile scrolling outside Telegram

[Sun-sunshine06]

Summary

• restore page-level scrolling for normal mobile browsers while keeping Telegram WebApp scroll lock
• add shared app-scroll-y containers for session list, chat thread, files, file view, settings, and new session
• tag the document with data-telegram-app during bootstrap so CSS can branch by environment

Test Plan

• bun run --cwd web typecheck
• bun run --cwd web build
• manually verify mobile scrolling outside Telegram on sessions, chat, files, and settings

Refs #133

[github-actions]

Findings

• None.

Summary
Review mode: initial. No issues found in modified lines. Residual risk: scroll behavior differences across Telegram vs non-Telegram; manual device verification recommended.

Testing

• Not run (automation).