Fix stale transfer telemetry and window fitting

[ClaudiuSchuster]

Summary

• use one process-owned, smoothed estimator for a matching single-file transfer and its queue
• reject implausible stale rclone speed/ETA pairs and return to a calculating state after traffic stops
• distinguish a retry that is demonstrably transferring from its preserved warning history
• keep extreme near-pause ETAs readable while retaining fuller values in tooltips
• fit dashboard height once per opening without compounding stale GTK overflow measurements
• document the telemetry and content-fitting guarantees

Validation

• git diff --check
• make check
• NO_AT_BRIDGE=1 ./tests/test-ui-widgets.sh --use-display
• Super-Linter 8.7.0 from the workflow-pinned image

Fixes #18
Fixes #20
Fixes #21