feat: add fixed tab number shortcuts

[hhhjin]

Summary

• add fixed Mod+1 through Mod+9 tab switching shortcuts in the desktop hotkey layer
• reserve those bindings in the store hotkey utilities and reject them in hotkey settings
• add coverage for tab-number resolution and reserved-binding validation

Testing

• pnpm test:packages
• pnpm ts:check:desktop
• pnpm lint:fix

Notes

• existing saved custom Mod+1 through Mod+9 bindings are not migrated in this patch

[gemini-code-assist]

Code Review

This pull request introduces fixed tab switching shortcuts (Mod+1 through Mod+9) and prevents these reserved bindings from being overridden in the application settings. The changes include a new utility for tab lookup, UI integration for the shortcuts, and validation logic in the store. Feedback suggests hardening the reservation check to include 'Ctrl' prefixes for cross-platform consistency, optimizing the tab activation callback to avoid unnecessary re-renders, and importing the reservation utility in the UI component.