feat: hierarchical codeplug projects + revamped import/export UI

[pskillen]

Problem

The initial genericise-import work (#7) introduces internal data models, a central store, and a multi-file/directory import UI, but it still treats the working state as a single, implicit codeplug. As the toolset grows (map, report, export #8, CRUD), users will want to keep several codeplugs side by side — e.g. "home repeaters", "contest weekend", "travel" — and switch between them without re-importing each time.

Intended outcome

Make the codeplug a first-class, switchable project. The app maintains multiple codeplug projects; one is active at a time, and all tools operate within the context of the active project.

• Project lifecycle: start a new (empty) codeplug, or start by importing an existing CPS export (files or whole directory) into a new project.
• Project switcher: UI to list, select, rename, duplicate, and delete codeplug projects; clearly show which project is active.
• Active-project context: the map (and future report/export/CRUD tools) read and write the active project's models only.
• Revamped import/export UI: a coherent, hierarchical surface where import (feat: genericise CPS import via internal data models #7) and export (feat: CPS export support (internal models → vendor format) #8) live per-project, rather than ad-hoc dropzones in a single tool's sidebar.

Affected tool(s)

• Shared state/storage layer and a new project-management UI consumed by all tools/ (map first; report/export/CRUD later).

Notes / dependencies

• Builds on feat: genericise CPS import via internal data models #7 (internal data models + central codeplug store). feat: genericise CPS import via internal data models #7 ships first as-is; this ticket generalises the single-codeplug store into a multi-project store.
• Pairs with feat: persist state in browser LocalStorage #9 (LocalStorage persistence): persistence should store the set of projects + the active project id, within LocalStorage size limits (document limits; consider IndexedDB if projects get large).
• Pairs with feat: CPS export support (internal models → vendor format) #8 (export): export operates on the active project.
• Privacy: projects are browser-local only; never commit operator codeplugs. Mapbox token stays in LocalStorage only.
• SPA (Vite + React + TS), not static HTML/JS.

Out of scope

• Cloud sync / multi-device sharing of projects.
• CRUD editing of codeplug contents (separate tickets).

[pskillen]

this is done