Add label manager for demo messages

[eleven-smg]

Summary

Adds a label manager for the Demo Admin Dashboard so maintainers can create, apply, and clean up labels across demo messages. It lives in a self-contained labels/ module under src/features/demo-admin-dashboard/, following the same structure as the existing snooze/ and templates/ features.

What's included

• labels/types.ts — DemoLabel, LabeledDemoMessage, and LabelUsage types.
• labels/labelNormalization.ts — pure helpers for name normalization, stable id generation, add/remove, usage counts, and unused-label detection.
• labels/labelFixtures.ts — deterministic demo labels and labeled messages using the reserved demo sender handles.
• labels/LabelManager.tsx — UI for creating labels, applying or removing them per message, reviewing usage counts, and surfacing unused labels for cleanup.
• labels/labelNormalization.test.ts — unit coverage for the helpers.
• labels/README.md — short overview of the module.
• Exposed the public API from the feature index.ts.

Notes

• All data is deterministic and synthetic; nothing references real mail, addresses, or secrets.
• Label ids are derived from names, so they stay stable and case-insensitive.

Closes #185