feat(app): add minimal RTL support

[opencode-agent]

Summary

• set the document direction to RTL for Arabic, Urdu, and Pakistani Punjabi (Shahmukhi), and restore LTR for every other locale
• mirror the main and legacy sidebar/session shell with logical positioning, borders, corners, resize handles, and direction-aware transitions
• let user and assistant prose determine direction automatically, with plaintext bidi handling for mixed-direction blocks
• keep terminal, shell output, source/diff viewers, file trees, paths, inline code, and diff counts explicitly LTR and isolated
• preserve all English copy and translation keys
• add no tests, as requested

Validation

• app typecheck
• Session UI typecheck
• UI typecheck
• production app build with Bun/Vite
• generated Tailwind output contains the RTL transform and logical corner/positioning utilities
• git diff --check
• verified no English dictionary changes
• real-app session timeline smoke passed for English, Arabic, and Urdu at 1400 × 900
• verified RTL document direction, automatic prose direction, and LTR code isolation; screenshots shared in Slack

Requested by: @Hona (Luke Parker via Slack)