feat(ui): revit link browser — element↔spec navigation

[thewrz]

Context

Spec writers need to navigate model elements ↔ linked specs/paragraphs in both directions, and see what is unlinked. UI consumer of the link inventory read model.

Scope

Create:

• UI page listing model elements ↔ linked specs/paragraphs with filter/search
• Drill-through to spec tree view (feat(ui): Phase 5b — spec tree viewer + parse job progress UI #39)
• Unlinked items visually flagged

Acceptance

• Element list ↔ spec list pivots both directions
• Unlinked items visually flagged

Blocked by

#38, #103

Out of scope

Editing mappings from the browser (follow-up).