docs: document file transfer protocol (#301)

docs/file-transfer-protocol.md

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+# FilePizza File Transfer Protocol
+
+This document explains the message-based protocol that FilePizza uses to
+transfer files directly between browsers over a WebRTC data channel.  It
+covers the complete conversation required to build either an uploader or a
+downloader and includes examples for common scenarios.
+
+## Architecture Overview
+
+```mermaid
+flowchart LR
+    Uploader -- WebRTC / PeerJS --> Downloader
+    Uploader -- REST --> Server[(FilePizza Server)]
+    Downloader -- REST --> Server
+    Server -- signalling / slug --> Uploader
+    Server -- signalling / slug --> Downloader
+```
+
+1. The uploader creates a channel with the server and receives a slug that
+   encodes its PeerJS identifier.
+2. The downloader resolves the slug via the server to obtain the uploader's
+   PeerJS identifier.
+3. All subsequent messages travel directly between peers over a reliable
+   WebRTC data channel.
+
+## Message Types
+
+Every message is a JSON object with a `type` field that matches one of the
+values in the table below.  Fields marked with `?` are optional.
+
+```mermaid
+classDiagram
+    class RequestInfo {
+        +"RequestInfo" type
+        +string browserName
+        +string browserVersion
+        +string osName
+        +string osVersion
+        +string mobileVendor
+        +string mobileModel
+    }
+    class Info {
+        +"Info" type
+        +FileInfo[] files
+    }
+    class FileInfo {
+        +string fileName
+        +number size
+        +string type
+    }
+    class Start {
+        +"Start" type
+        +string fileName
+        +number offset
+    }
+    class Chunk {
+        +"Chunk" type
+        +string fileName
+        +number offset
+        +ArrayBuffer bytes
+        +boolean final
+    }
+    class ChunkAck {
+        +"ChunkAck" type
+        +string fileName
+        +number offset
+        +number bytesReceived
+    }
+    class Pause {
+        +"Pause" type
+    }
+    class Done {
+        +"Done" type
+    }
+    class Error {
+        +"Error" type
+        +string error
+    }
+    class PasswordRequired {
+        +"PasswordRequired" type
+        +string errorMessage?
+    }
+    class UsePassword {
+        +"UsePassword" type
+        +string password
+    }
+    class Report {
+        +"Report" type
+    }
+```
+
+Chunks are sent in pieces of at most 256 KiB (`MAX_CHUNK_SIZE`). The `final` flag in a `Chunk` message marks the last piece of a file.
+
+## Normal Transfer Sequence
+
+The following diagram shows the exchange for downloading multiple files
+without a password.
+
+```mermaid
+sequenceDiagram
+    participant D as Downloader
+    participant U as Uploader
+    D->>U: RequestInfo
+    U-->>D: Info(files)
+    loop For each file
+        D->>U: Start(fileName, offset=0)
+        loop For each chunk
+            U-->>D: Chunk(offset, bytes, final=false)
+            D->>U: ChunkAck(offset, bytesReceived)
+        end
+        U-->>D: Chunk(offset, bytes, final=true)
+        D->>U: ChunkAck(offset, bytesReceived)
+    end
+    D->>U: Done
+    U-->>D: close connection
+```
+
+## Password‑Protected Transfers
+
+If the uploader specified a password when creating the channel, the
+conversation includes an authentication step.
+
+```mermaid
+sequenceDiagram
+    participant D as Downloader
+    participant U as Uploader
+    D->>U: RequestInfo
+    U-->>D: PasswordRequired(errorMessage?)
+    D->>U: UsePassword(password)
+    U-->>D: Info(files) or PasswordRequired("Invalid password")
+    Note over D,U: Continue with normal transfer sequence on success
+```
+
+## Pause and Resume
+
+A downloader may pause an in‑progress transfer.  To resume, it reconnects and
+requests the remainder of the file starting at the last acknowledged offset.
+
+```mermaid
+sequenceDiagram
+    participant D as Downloader
+    participant U as Uploader
+    D->>U: Start(fileName, offset=0)
+    U-->>D: Chunk(...)
+    D->>U: ChunkAck(...)
+    D->>U: Pause
+    Note over D,U: Connection closed or kept idle
+    D->>U: Start(fileName, offset=previouslyAcked)
+    Note over D,U: Transfer resumes from offset
+```
+
+## Reporting
+
+A special PeerJS connection with metadata `{ type: "report" }` causes the
+uploader to broadcast a `Report` message to all connected downloaders and to
+redirect its own UI to a reported page.  Downloaders receiving this message
+should abort the transfer.
+
+```mermaid
+sequenceDiagram
+    participant Reporter
+    participant U as Uploader
+    participant D as Downloader
+    Reporter->>U: Peer connection(type="report")
+    U-->>D: Report
+    U-->>Reporter: redirect to /reported
+```
+
+## Example Conversations
+
+### Single file without password
+
+```
+RequestInfo
+Info [{ fileName: "photo.jpg", size: 1048576, type: "image/jpeg" }]
+Start { fileName: "photo.jpg", offset: 0 }
+Chunk { offset: 0, bytes: <256 KB>, final: false }
+ChunkAck { offset: 0, bytesReceived: 262144 }
+...
+Chunk { offset: 1048576, bytes: <0>, final: true }
+ChunkAck { offset: 1048576, bytesReceived: 0 }
+Done
+```
+
+### Password‑protected download
+
+```
+RequestInfo
+PasswordRequired
+UsePassword { password: "secret" }
+Info [...]
+...
+```
+
+### Resuming after interruption
+
+```
+RequestInfo
+Info [...]
+Start { fileName: "video.mp4", offset: 0 }
+Chunk/ChunkAck exchanges...
+<connection drops after 1 MB>
+Start { fileName: "video.mp4", offset: 1048576 }
+Chunk/ChunkAck exchanges...
+Done
+```
+
+---
+
+With these message definitions and sequences you can implement a compatible
+uploader or downloader for FilePizza or adapt the protocol for other
+applications.