FIX: Repair mobile chat responsiveness

[romanlutz]

Description

At 390x844, opening Attack Conversations left only about 50px for chat, while wide media previews overflowed their message bubbles and forced horizontal scrolling. This change preserves a usable chat surface on narrow screens without changing the desktop workflow.

• Render Attack Conversations as a Fluent UI modal end drawer at 600px and below, while retaining the 280px inline panel on desktop.
• Keep historical mobile drawers closed until requested, close them after conversation selection or creation, and restore focus to the toggle after dismissal.
• Constrain image, video, and audio previews to the available message-bubble width while preserving aspect ratios and desktop size caps.
• Add accessible drawer state wiring and responsive component and browser regression coverage.

Conversation panel at 390x844

Before	After

Image message at 390x844

Before	After

Tests and Documentation

• TypeScript type-check passed.
• Targeted ESLint passed for all changed frontend files.
• ChatWindow and MessageList Jest suites passed: 128 tests.
• Targeted Playwright tests passed for the mobile drawer, responsive image, and responsive audio flows.
• Documentation: N/A; no public API or workflow documentation changed.
• JupyText: N/A; no documentation notebooks changed.