Show routing/hop info on messages

[MrSurly]

Summary

Show each message's routing info (direct vs. relayed, hop count) in the chat UI. This data is already captured from firmware but never surfaced.

What's already there

• Firmware sends hop count with every received message: path_len in both the direct-message and channel-message V3 response formats (lib/ble/ble_responses.dart:501,531, SNR is sent alongside it too).
• It's parsed into pathLength/hopCount and persisted per-message in the messages table (lib/database/tables.dart:28-29: isDirect "True if 0 hops" and hopCount "Number of relay hops (-1 = unknown)"), written in lib/repositories/message_repository.dart (e.g. lines 752, 771, 1424, 2070).
• SNR is parsed (response.snr) but is not currently persisted per-message the way hop count is.

What's missing

None of this reaches the UI. There's no reference to .hopCount or .snr anywhere under lib/screens or lib/widgets -- the data is captured and stored, then discarded from the user's perspective.

Desired behavior

• Show a small indicator per message (e.g. in the existing per-message status-icon row added in the Add status icons to chat screens #46 status-icons work) indicating whether it arrived direct or via N hops.
• Consider also surfacing SNR if it's cheap to persist alongside hop count (may need a migration to add an snr column to messages, mirroring the existing snr column already on the contact_position_histories table at lib/database/tables.dart:195).
• A tap/long-press on the indicator could show more detail (exact hop count, SNR) for users who want it, keeping the default view unobtrusive.

Open questions

• Exact visual treatment (icon + tooltip vs. inline text) -- should follow whatever pattern the Add status icons to chat screens #46 status-icons work already established for consistency.
• Whether SNR is worth the schema migration for v1, or whether hop count alone covers the ask.