Add mobile Shift+Tab hotkey

[ZacharyZcR]

Overview

Add Shift as a mobile terminal keyboard modifier and expose Shift+Tab as a selectable hotkey.

• Added: Shift modifier key to mobile keyboard definitions and preset top bars
• Added: Shift+Tab shortcut key using the reverse-tab terminal sequence
• Updated: top keyboard modifier state to track Ctrl, Alt, and Shift
• Updated: terminal input handling so Shift+Tab sends \x1b[Z

Changes Made

• Add shift and shiftTab to ALL_KEYS so they appear in keyboard customization.
• Include Shift in the default, minimal, developer, sysadmin, and compact top bars.
• Include Shift+Tab in shortcut rows across presets.
• Let the top KeyboardBar toggle Shift and send reverse-tab when Shift is active and Tab is pressed.
• Let the full custom keyboard send Shift+Tab directly and support Shift+Tab when a Shift key is present.
• Extend session modifier handling to include Shift for mobile text input.

Related Issues

• Closes [FEATURE] Add Shift+Tab as a hotkey to Mobile Support#529

Screenshots / Demos

Not included; this should be verified on Android/iOS by opening the terminal keyboard and sending Shift+Tab to an SSH session.

Validation run locally:

• npx eslint app/tabs/sessions/terminal/keyboard/KeyDefinitions.ts app/tabs/sessions/terminal/keyboard/KeyboardBar.tsx app/tabs/sessions/terminal/keyboard/CustomKeyboard.tsx app/tabs/sessions/Sessions.tsx passes with existing Sessions.tsx warnings only.
• npx prettier --check app/tabs/sessions/terminal/keyboard/KeyDefinitions.ts app/tabs/sessions/terminal/keyboard/KeyboardBar.tsx app/tabs/sessions/terminal/keyboard/CustomKeyboard.tsx app/tabs/sessions/Sessions.tsx passes.
• git diff --check passes.
• npx tsc --noEmit still fails on existing repository-wide TypeScript errors outside this change.

Checklist

• Code follows project style guidelines
• Supports mobile and desktop UI/app (if applicable)
• I have read Contributing.md