feat: Add presence heartbeat for Matrix online status

- matrix-gateway: POST /internal/matrix/presence/online endpoint
- usePresenceHeartbeat hook with activity tracking
- Auto away after 5 min inactivity
- Offline on page close/visibility change
- Integrated in MatrixChatRoom component

node_modules/fast-json-stable-stringify/.eslintrc.yml

@@ -0,0 +1,26 @@
+extends: eslint:recommended
+env:
+  node: true
+  browser: true
+rules:
+  block-scoped-var: 2
+  callback-return: 2
+  dot-notation: 2
+  indent: 2
+  linebreak-style: [2, unix]
+  new-cap: 2
+  no-console: [2, allow: [warn, error]]
+  no-else-return: 2
+  no-eq-null: 2
+  no-fallthrough: 2
+  no-invalid-this: 2
+  no-return-assign: 2
+  no-shadow: 1
+  no-trailing-spaces: 2
+  no-use-before-define: [2, nofunc]
+  quotes: [2, single, avoid-escape]
+  semi: [2, always]
+  strict: [2, global]
+  valid-jsdoc: [2, requireReturn: false]
+  no-control-regex: 0
+  no-useless-escape: 2