feat: edit standalone images through source projection

[Azganoth]

Summary

• Replace the detached image Markdown input and duplicate parsing path with the shared in-document source projection engine, including parser-backed edits, projection-local history and clipboard behavior, and authored-target preservation when rendered images move.
• Keep standalone images and links whose complete label is one image mounted beneath a 28-pixel source line, with pointer entry at the alt text, directional keyboard entry, clean boundary exit, and short path-free unavailable states.
• Update the product specification, architecture record, changelog, focused editor coverage, and desktop E2E expectations for the final interaction model.

Related Issue

Closes #313

Verification

• pnpm exec vitest run src/features/editor/tests/sourceProjectionImage.test.ts src/features/editor/tests/sourceProjectionImageLink.test.ts src/features/editor/plugins/imageView.test.ts — verifies retained standalone and linked images, pointer and keyboard entry, boundary exit, valid and literal commits, history, clipboard behavior, and unavailable states.
• pnpm check:frontend — verifies formatting, TypeScript and Oxlint checks, the production frontend build, and the complete frontend test suite with coverage thresholds.

Not verified: Windows desktop E2E and manual application interaction were not run locally.

Notes

The projection deliberately adds one 28-pixel source line before the retained image while editing. This keeps source as native document text with ordinary ProseMirror selection behavior; avoiding that temporary layout shift would require returning to an overlay or detached control model.

Structured image controls, including file replacement, were deferred to the Design structured image editing controls Project draft rather than coupled to source projection. Links with mixed text, multiple images, or other label content keep the existing source-only whole-link projection.