docs: overhaul project documentation and contributing guide (Task #190)

docs/functions.md

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+# ⚙️ Functions & API
+
+Plexus communicates primarily through WebSockets (Socket.io) for real-time interaction.
+
+## 🔌 Socket Events
+
+### 📥 Client to Server
+- `join({ walletAddress, username })`: Register or login a user.
+- `sendMessage({ channelId, walletAddress, content, txId })`: Send a message to a channel.
+- `toggleReaction({ messageId, walletAddress, emoji })`: Toggle a reaction on a message.
+- `updateUsername({ walletAddress, newUsername, txId })`: Change username (simulated cost).
+- `getProfile(walletAddress)`: Fetch user profile and wall posts.
+- `updateProfile({ walletAddress, bio, bannerColor })`: Update profile details.
+- `createPost({ walletAddress, content })`: Post a message to the user's wall.
+
+### 📤 Server to Client
+- `userList(users)`: Broadcast the updated list of online/offline users.
+- `newMessage(message)`: Broadcast a new message to all clients.
+- `updateReactions({ messageId, reactions })`: Broadcast updated reactions for a message.
+- `usernameUpdated({ username })`: Confirm a username change to the specific user.
+- `profileData(data)`: Send requested profile data to a user.
+- `profileUpdated(data)`: Confirm profile update.
+- `postCreated(post)`: Confirm post creation.
+- `error({ message })`: Send error messages to the client.
+
+## 🌐 REST API
+- `GET /api/channels`: List all available chat channels.
+- `GET /api/messages/:channelId`: Fetch the last 100 messages and reactions for a channel.
+
+## 🛠 Internal Logic
+- **Username Authority**: The server validates and ensures unique usernames, appending suffixes if necessary.
+- **Transaction Simulation**: The client simulates a 1.5s delay and a 5% failure rate for "blockchain" transactions.