Restructure project into feature/domain/shared areas, add path aliases and UI shell components

[DMNerd]

Motivation

• Split the codebase into logical ownership boundaries (app, features, shared, domain) to improve modularity and make public feature APIs explicit.
• Introduce scoped import aliases so new code imports feature/public APIs rather than reaching into internals.
• Centralize shared utilities and UI primitives and extract stage/panel shell pieces to simplify App composition.

Description

• Reorganized & renamed many files into src/features/*, src/domain/*, and src/shared/*, and added feature index entrypoints (for example @features/display, @features/instrument, @features/export, @features/practice, @features/theory, @features/fretboard, @features/share).
• Replaced many relative/legacy imports with scoped aliases and updated runtime build resolution by adding path aliases to tsconfig.json and vite.config.js (@app/*, @features/*, @shared/*, @domain/*, @styles/*, and kept @/* as fallback).
• Extracted UI orchestration/shell components from App into new modules: AppPanels, AppModals (in src/app/shell/AppPanels.jsx), StageShell (in src/app/shell/StageShell.jsx), and ToastProvider (in src/app/providers/ToastProvider.jsx), and updated App.jsx to use them.
• Moved and centralized shared libs and UI primitives into src/shared/lib/* and src/shared/ui/*, moved pure domain music-theory modules into src/domain/*, and updated many store/hooks/components to use the new locations.
• Added docs/project-structure.md documenting the new organization and import boundary rules.
• Updated configuration and tooling entries: knip.json entry glob extended to include .{js,jsx,ts,tsx} tests, and many tests/files were relocated to mirror the new structure under src/tests/**.

Testing

• Updated and ran the unit test suite via npm test which exercised the renamed modules and tests paths; the test suite completed successfully.
• Performed a TypeScript check with tsc --noEmit against the updated tsconfig.json to validate path aliases and types; the typecheck completed successfully.
• Sanity-checked the development build/resolution using the updated Vite aliases by starting a local dev server (npm run dev) and confirming the app bootstraps without missing-module errors.

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Codex Task