Add Markdown document templates

[colinchambachan]

Summary

• Adds global Markdown document templates to General settings with Add/Edit/Delete flows.
• Supports deterministic placeholder expansion for {{title}}, {{filename}}, {{date}}, {{time}}, and {{datetime}}.
• Adds an editor-scoped Insert Template menu for Markdown edit tabs; templates append to existing content instead of replacing it.
• Keeps existing Markdown creation flows unchanged; this PR does not add a new Explorer or file-creation surface.

Closes #1222

Screenshots / Recording

Screen.Recording.2026-04-30.at.8.50.42.AM.mov

Testing

• pnpm lint
• pnpm typecheck
• pnpm test
• pnpm build

Note: local build exits 0 but prints the existing /usr/local/bin/orca-dev symlink permission warning.

AI Code Review Summary

I reviewed this change for scope, cross-platform behavior, and security-sensitive paths. The implementation keeps templates in app settings and inserts rendered Markdown through the existing editor draft/save path, so it does not introduce new filesystem write IPC or alternate document-creation behavior.

Cross-platform check: no new keyboard shortcuts were added. Placeholder dates use local Date fields instead of toISOString(), avoiding timezone shifts. Filename/title extraction normalizes path separators for display/template variables only and does not introduce security-sensitive path handling.

Security audit: template content is user-authored Markdown inserted into the user's current document. Unknown placeholders are preserved rather than evaluated. Persisted template data is normalized on load/update so malformed stored values do not leak into renderer state. No new file overwrite path, absolute path construction path, or traversal-sensitive IPC was added.

Platform Notes

Tested locally on macOS with Node v22.22.2. Orca targets macOS, Linux, and Windows; this change avoids platform-specific shortcuts and filesystem assumptions.

Follow-ups Intentionally Out of Scope

• Repo-local templates
• File-backed template discovery
• Template import/export
• Custom user-defined variables
• New Markdown creation surfaces