Visor: keep the strip above Android Chrome's bottom controls

[lannbot]

Summary

• Size the drawer with svh, the stable viewport with browser toolbars visible, instead of vh.
• Cap the drawer to reserve the 56px strip plus 96px below it, including short landscape screens.
• Size the page with 100dvh so the app area follows the currently available viewport.

Verification

• Astra review completed.
• just check, workspace Rust tests and Deno tests passed.
• Full Chromium suite passed: 29 scenarios, including a new mobile-context test at 390x844, 360x640, 320x480 and 844x390.
• Regression checks cover viewport-unit rules, strip and app-area geometry, no shell overflow, and scrolling the final control into view.
• Physical Android toolbar verification was not available; Playwright does not emulate toolbar expansion/collapse.

Automerge will be armed.