feat(editor): decompose and nudge components, delete anchors

[kostyafarber]

Summary

• Decompose Components: new command in the Glyph menu and the canvas context menu. The context-menu item is shown only when the selection is entirely components. Uses the existing Rust decompose intent through Editor.decomposeSelection().
• Nudge components: arrow keys move selected components with the standard 1/10/100 increments, reusing the component drag edit.
• Delete anchors: Delete, Backspace, and Edit → Delete remove anchor-only selections. Mixed point-and-anchor selections still refuse, as before.
• Each edit applies to every source selected for editing as one undo step. With one source selected, only the active source changes. Deleting and decomposing components share one helper in Editor.

Issue

Refs #395 — fills gaps left by component authoring; no issue tracks these three directly.

Testing

• pnpm check passed (lint, typecheck, browser check, dead code, full vitest suite).
• New tests: component nudge with undo (SelectNudge.test.ts), decompose through the renderer command with undo plus refusal on non-component selections (ComponentEditing.test.ts), anchor deletion with undo (Deletion.test.ts).
• Multi-source behavior is covered by the shared target resolution already tested for component deletion; there is no new multi-source test for these three edits.
• Not yet checked by hand in the running app. E2E not run locally; left to PR CI.