docs: render architecture & flow diagrams as draw.io charts

Replace ASCII-art diagrams across the README and docs/ with editable
.drawio sources plus exported PNGs, so the diagrams render clearly in
rendered markdown and can be maintained/regenerated instead of being
hand-edited as fragile text art. Non-diagram blocks (code, folder
trees, UI wireframes) were left as text.

docs/plans/2026-03-22-primary-backup-data-connections-design.md

@@ -39,28 +39,8 @@ Both endpoints use the same `Protocol`. EF Core migration renames `Configuration
 
 The `DataConnectionActor` Reconnecting state is extended:
 
-```
-Connected
-    │ disconnect detected
-    ▼
-Push bad quality to all subscribers
-    │
-    ▼
-Retry active endpoint (5s interval)
-    │ failure
-    ▼
-_consecutiveFailures++
-    │
-    ├─ < FailoverRetryCount → retry same endpoint
-    │
-    ├─ ≥ FailoverRetryCount AND backup exists
-    │     → dispose adapter, switch _activeEndpoint, reset counter
-    │     → create fresh adapter with other config
-    │     → attempt connect
-    │
-    └─ ≥ FailoverRetryCount AND no backup
-          → keep retrying indefinitely (current behavior)
-```
+![primary-backup-failover-state-machine](diagrams/primary-backup-failover-state-machine.png)
+<!-- source: diagrams/primary-backup-failover-state-machine.drawio — edit, then re-export with export-drawio.sh -->
 
 **On successful reconnect (either endpoint):**
 1. Reset `_consecutiveFailures = 0`