[desktop] Fix module-level state and silent failure in caret plugin

[Copilot]

caret.ts used module-level activeView/activeEl variables, meaning each new editor instance silently overwrote the previous ones. Additionally, missing parentElement was swallowed via optional chaining, producing an invisible, never-appended caret element with no error surfaced.

Changes

• Multi-instance tracking – replaced activeView/activeEl with Map<EditorView, HTMLElement>; setCaretStyle now iterates all active instances instead of clobbering on each mount

// before
let activeView: EditorView | null = null;
let activeEl: HTMLElement | null = null;

// after
const activeInstances = new Map<EditorView, HTMLElement>();

• Explicit failure on missing parent – replaced view.dom.parentElement?.appendChild(el) with a guard that throws an actionable error, so a mis-mounted editor fails loudly at init rather than silently rendering nothing
• Lifecycle cleanup – destroy() now calls activeInstances.delete(view) (keyed deletion), replacing the previous null-assignment that broke any remaining instances

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Greptile Summary

Fixed critical bugs in caret plugin by replacing module-level singleton state with a Map<EditorView, HTMLElement> to properly track multiple editor instances, and replaced silent optional chaining failure with explicit error when parent element is missing.

Key improvements:

• Multi-instance support - setCaretStyle now updates all active editors instead of only the most recent one
• Fail-fast validation - missing parent element now throws descriptive error at initialization instead of silently creating an orphaned DOM element
• Proper cleanup - destroy() uses keyed deletion from Map instead of nulling shared state

Confidence Score: 5/5

• This PR is safe to merge with no risk
• The changes are well-architected bug fixes that improve code correctness without introducing new complexity. The refactoring from singleton to Map properly handles multiple instances, and the explicit error handling replaces silent failures with actionable errors.
• No files require special attention

Important Files Changed

Filename	Overview
apps/desktop/src/editor/plugins/caret.ts	Refactored from module-level singleton to Map-based multi-instance tracking; added explicit parent element validation

_{Last reviewed commit: ba5fb5b}