[gx1400]

Closes #454. Stacked on #462. It depends on #460 for metrics.db.

Engine:

• Per-source counters: the engine keeps a second DashboardCounters per source, the same class as the whole feed's. Every packet a source lists is counted there.
• Copies count for each source: a copy another source brought first (today only RecordSource) is summarized and counted for this source. All sources still counts it once.
• GetSourceDashboardAsync(range, sourceId, filter) works for every range, including 7 days from that source's hours. Farthest, weather, the tables and heard on RF stay the whole feed's.
• Sources compared: Dashboard.Comparison (DashboardSourceRow: stations, packets, heard only here) is on the All sources dashboard when there are two or more sources. Over 7 days it covers the last 24 hours.
• Restarts: per-source counters are saved in dashboard-history.json (HistorySnapshot.Sources, optional, so older files still load) and hourly in metrics.db.
• Schema change: metrics.db moves to schema 2 with a source column. A schema 1 file from feat(dashboard): a 7 days range, from hours kept 90 days #460 builds is set aside and started afresh; none was ever released.
• Clearing: clearing the packets clears the per-source counters too.

UI:

• A Source picker beside the range, only when the profile has two or more sources. It's kept for the session, like the range; neither is saved.
• A Sources compared tile.
• The note under the range says which tiles still count every source.

Spec: F1.22 behavior 7a and the service surface.

Tests:

• An engine test with two connected sources: each source's packets and stations, the copy counted for both, and the comparison.
• A restart test: a source's counts and its 7-day hours come back.
• A view-model test: picker, comparison rows, asking for one source, the note.

Release build, all tests and csharpier pass locally.