Desktop: add snapshot inspection and stored observation administration

[Dumdart]

What changed

• add shared desktop snapshot presentation models, health rendering, topic-state badges, filtering controls, and confirmed Reconnect & observe
• add application-wide retention presets, lossless byte/age editors, validation, and policy enforcement previews
• add broker/global cache usage and persisted-topic inspection without exposing payload contents
• add exact-ID deletion plans, detailed partial results, stale-preview rejection, and active/inactive hydrated-state reconciliation
• make automatic unsubscribed cleanup operational after subscription removal or filter changes
• add metadata-only administration logging and coherent snapshot refresh after mutations

Why

The desktop needed one coherent inspection surface for current broker state and a safe control center for persisted MQTT observations. Destructive policy and cache operations now require an exact preview and preserve live or concurrently replaced observations.

User impact

Users can inspect snapshot freshness and completeness, tune persisted-observation retention, review storage usage, and delete selected, unsubscribed, broker-scoped, or global cached observations. Reconnect & observe explicitly warns that it interrupts and renews the active broker connection.

Validation

• uv run pytest — 267 passed
• uv build — source distribution and wheel built successfully
• git diff --check — clean
• incremental Graphify code graph updated