Refactor TODONT module with configurable hotkey and bulk archive #16

Problem

The TODONT/Archive module has several limitations:

• The Archive hotkey (Cmd/Ctrl+Shift+Enter) is hardcoded with no way to customize it
• Archiving doesn't work on multi-selected blocks
• The ARCHIVED button styling replaces the button's inner text, which breaks accessibility (screen readers see "x" instead of "ARCHIVED")
• The module uses a raw keydown listener instead of Roam's command palette, so it doesn't appear in the command search
• Missing null-safety guards in replaceText and mobile icon handlers

Proposed Changes

• Accept extensionAPI parameter in initializeTodont, return { toggle, cleanup } for proper lifecycle management
• Register Archive command via extensionAPI.ui.commandPalette.addCommand with configurable default hotkey
• Add archiveBlock helper using updateBlock for multi-select support
• Use CSS ::before pseudo-element for ARCHIVED button styling (keeps text as "ARCHIVED" for accessibility)
• Replace styleArchivedButtons with syncArchivedButton/syncArchivedButtons pattern
• Add null-safety guards and proper cleanup on extension unload