feat: Enhance dashboard with filtering options and sync SLM status

- Added a new filtering system to the dashboard for device types and statuses.
- Implemented asynchronous SLM status synchronization to update the Emitter table.
- Updated the status snapshot endpoint to sync SLM status before generating the snapshot.
- Refactored the dashboard HTML to include filter controls and JavaScript for managing filter state.
- Improved the unit detail page to handle modem associations and cascade updates to paired devices.
- Removed redundant code related to syncing start time for measuring devices.

backend/services/snapshot.py

@@ -146,6 +146,22 @@ def emit_status_snapshot():
                     "coordinates": "",
                 }
 
+        # --- Derive modem status from paired devices ---
+        # Modems don't have their own check-in system, so we inherit status
+        # from whatever device they're paired with (seismograph or SLM)
+        for unit_id, unit_data in units.items():
+            if unit_data.get("device_type") == "modem" and not unit_data.get("retired"):
+                roster_unit = roster.get(unit_id)
+                if roster_unit and roster_unit.deployed_with_unit_id:
+                    paired_unit_id = roster_unit.deployed_with_unit_id
+                    paired_unit = units.get(paired_unit_id)
+                    if paired_unit:
+                        # Inherit status from paired device
+                        unit_data["status"] = paired_unit.get("status", "Missing")
+                        unit_data["age"] = paired_unit.get("age", "N/A")
+                        unit_data["last"] = paired_unit.get("last")
+                        unit_data["derived_from"] = paired_unit_id
+
         # Separate buckets for UI
         active_units = {
             uid: u for uid, u in units.items()