rename: kafka → samsa

Full project rename from kafka to samsa (after Gregor Samsa, who
woke one morning from uneasy dreams to find himself transformed).

- Module: github.com/metamorphosis-dev/kafka → samsa
- Binary: cmd/kafka/ → cmd/samsa/
- CSS: kafka.css → samsa.css
- UI: all 'kafka' product names, titles, localStorage keys → samsa
- localStorage keys: kafka-theme → samsa-theme, kafka-engines → samsa-engines
- OpenSearch: ShortName, LongName, description, URLs updated
- AGPL headers: 'kafka' → 'samsa'
- Docs, configs, examples updated
- Cache key prefix: kafka: → samsa:

config.example.toml

@@ -1,4 +1,4 @@
-# kafka configuration
+# samsa configuration
 # Copy to config.toml and adjust as needed.
 # Environment variables are used as fallbacks when a config field is empty/unset.
 
@@ -10,13 +10,13 @@ port = 8080
 http_timeout = "10s"
 
 # Public base URL for OpenSearch XML (env: BASE_URL)
-# Set this so browsers can add kafka as a search engine.
+# Set this so browsers can add samsa as a search engine.
 # Example: "https://search.example.com"
 base_url = ""
 
 # Link to the source code (shown in footer as "Source" link)
-# Defaults to the upstream kafka repo if not set.
-# Example: "https://git.example.com/my-kafka-fork"
+# Defaults to the upstream samsa repo if not set.
+# Example: "https://git.example.com/my-samsa-fork"
 source_url = ""
 
 [upstream]