[6.x] Restructure frontend: adopt common/ + modules/ pattern

[brianjhanson]

Summary

Reorganizes resources/js/ following the Spatie Laravel/Inertia frontend structure, adapted for Vue 3.

New structure

resources/js/
├── common/          # Generic, domain-agnostic code
│   ├── components/  # UI primitives
│   ├── composables/ # Generic Vue composables
│   ├── form/        # Generic form inputs
│   ├── layouts/     # App layouts (was layout/)
│   ├── types/       # Shared types
│   └── utils/
├── modules/         # Feature-specific domains
│   ├── admin-table/
│   ├── auth/
│   ├── entry-types/
│   ├── install/
│   ├── navigation/
│   ├── permissions/
│   ├── plugin-manager/
│   ├── sections/
│   ├── settings/
│   ├── sites/
│   ├── updater/
│   ├── user/
│   └── utilities/
└── pages/           # Inertia pages, reorganized into subdirs
    ├── auth/
    ├── graphql/
    ├── install/
    ├── settings/
    ├── updater/
    └── utilities/

Key changes

• common/ vs modules/: The guiding question — does it relate to a domain or feature of the app? Yes → modules/, No → common/
• Page names deduplicated: Directory provides context, filename only expresses what's unique within it. SettingsEmailPage.vue → settings/Email.vue, LoginPage.vue → auth/Login.vue, etc.
• Module composables colocated: useEditableTable, useServerPagination, etc. live inside modules/admin-table/composables/ rather than a global flat directory
• createCraftColumnHelper moved to utils/: It's a helper function, not a component — lives in modules/admin-table/utils/ (note: currently helpers/, see follow-up)
• PHP Inertia keys updated: All Inertia::render() and ->inertiaPage() calls in src/Http/Controllers/ updated to use path-based keys (e.g. 'settings/Email')

Not in scope

• packages/craftcms-cp/ — Lit-based web component library, untouched
• resources/js/legacy.ts and legacy assets

Follow-up items

A few Vue convention adjustments worth a separate pass:

• Single-word component names (Badge, Modal, Date, Empty, Pane, Text, Select, etc.) violate Vue Style Guide Priority A
• AuthBase.vue → AuthLayout.vue for consistency with other layouts
• helpers/ → utils/ (JS/Vue convention)
• pages/install/Install.vue → pages/install/Index.vue for consistency with pages/updater/Index.vue

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