Refactor frontend architecture and stabilize local workflows

[sviat-barbutsa]

Summary

• split the graph, semantic, and system-store hotspots into focused typed modules
• make browser sessions, project switching, diagnostics, preferences, and lifecycle ownership explicit
• consolidate document/search editor behavior and strengthen shared accessible UI primitives
• stabilize seamless local preview across reloads, localhost, project changes, and multiple tabs

Validation

• TypeScript workspace build
• 316 passing tests, 0 failures (2 intentional Windows symlink skips)
• 41/41 frontend source tests
• 25/25 desktop contract tests
• 6/6 Windows Tauri/Rust tests
• production Vite build, bundle budgets, secretless scan, source-artifact check
• accessibility contract and browser smoke
• one-command local preview at both 127.0.0.1 and localhost
• manual multi-tab project/search/document Preview + Markdown flow