feat: viewer role with invite-based registration

[akifbayram]

Summary

Adds a viewer role to the existing admin/member permission model, along with invite-based registration and per-location default join role configuration.

Viewer Role

• New viewer role in location_members (schema updated with 3-way check constraint)
• Viewers can browse bins and scan QR codes but cannot create, edit, delete, pin, or upload photos
• Server-side enforcement via requireMemberOrAbove helper in bin/photo routes
• Client-side UI guards hide action buttons, bulk actions, and FAB for viewers
• usePermissions hook updated with canEdit / isViewer flags

Invite-Based Registration

• REGISTRATION_MODE env var replaces REGISTRATION_ENABLED (open | invite | closed)
• invite mode requires a valid location invite code during registration — user auto-joins that location
• Register page shows invite code field when mode is invite; hides register link when closed

Default Join Role

• default_join_role column on locations table (member or viewer)
• Configurable per-location in the retention/settings dialog
• New members joining via invite code receive the configured default role

Member Management

• Admins can change member roles (admin/member/viewer) in the members dialog
• Role badge and dropdown shown per member
• Invite QR code displayed in members dialog for easy sharing

Test Plan

• Register with REGISTRATION_MODE=invite using a valid invite code — verify auto-join with correct role
• Register with REGISTRATION_MODE=closed — verify registration is blocked
• Join a location with default_join_role=viewer — verify viewer role assigned
• As viewer: verify bins are visible but create/edit/delete/pin/photo actions are hidden
• As admin: change a member's role and verify it takes effect
• Verify existing admin/member flows are unaffected
• Run npx vitest run (client + server) — all tests pass