refactor: extract streaming engine into internal/flinger

Move the capture/encode/send pipeline out of cmd/teleportfling into a
GUI-free Engine so the CLI and the desktop app share one implementation:
- flinger: Config/Engine/Status, audio+video+stats loops, pattern source
- config: JSON persistence at ~/.config/teleportfling/config.json
- cmd/teleportfling: thin CLI wrapper around the engine (same flags)
- golangci: extend exclusions to flinger/config

.golangci.yml

@@ -27,7 +27,7 @@ linters:
       # Wire-format int conversions: the Teleport protocol defines 32-bit fields
       # on the wire (uint32) while the reference uses int32 struct fields. These
       # casts are intentional and mirror obs-teleport's types.go.
-      - path: "internal/protocol/"
+      - path: "(internal/protocol|internal/flinger)/"
         linters:
           - gosec
         text: "G115"
@@ -42,6 +42,10 @@ linters:
         linters:
           - gosec
         text: "G115"
+      # Config path comes from os.UserConfigDir(); safe to read/write.
+      - path: "internal/config/"
+        linters:
+          - gosec
 
 formatters:
   enable: