Import/Export backend prep

doc/importexport.md

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+# Import/Export
+
+## Main blocks
+
+This is basic overview of the main import/export blocks.
+
+```mermaid
+graph LR
+    S[ProtonMail server]
+    U[User]
+
+    subgraph "Import/Export app"
+        Users
+        Frontend["Qt / CLI"]
+        ImportExport
+        Transfer
+
+        Frontend --> ImportExport
+        Frontend --> Transfer
+        ImportExport --> Users
+        ImportExport --> Transfer
+    end
+
+    EML --> Transfer
+    MBOX --> Transfer
+    IMAP --> Transfer
+    S --> Transfer
+
+    Transfer --> EML
+    Transfer --> MBOX
+    Transfer --> S
+
+    U --> Frontend
+```
+
+## Code structure
+
+More detailed graph of main types used in Import/Export app and connection between them.
+
+```mermaid
+graph TD
+    PM[ProtonMail Server]
+    EML[EML]
+    MBOX[MBOX]
+    IMAP[IMAP]
+
+    subgraph "Import/Export app"
+        subgraph PkgUsers
+            subgraph PkgCredentials
+                CredStore[Store]
+                Creds[Credentials]
+
+                CredStore --> Creds
+            end
+
+            US[Users]
+            U[User]
+
+            US --> U
+        end
+
+        subgraph PkgFrontend
+            CLI
+            Qt
+        end
+
+        subgraph PkgImportExport
+            IE[ImportExport]
+        end
+
+        subgraph PkgTransfer
+            Transfer
+            Rules
+            Progress
+
+            Provider
+            LocalProvider
+            EMLProvider
+            MBOXProvider
+            IMAPProvider
+            PMAPIProvider
+
+            Mailbox
+            Message
+
+            Transfer --> |source|Provider
+            Transfer --> |target|Provider
+            Transfer --> Rules
+            Transfer --> Progress
+
+            Provider --> LocalProvider
+            Provider --> EMLProvider
+            Provider --> MBOXProvider
+            Provider --> IMAPProvider
+            Provider --> PMAPIProvider
+
+            LocalProvider --> EMLProvider
+            LocalProvider --> MBOXProvider
+
+            Provider --> Mailbox
+            Provider --> Message
+
+        end
+
+        subgraph PMAPI
+            APIM[ClientManager]
+            APIC[Client]
+
+            APIM --> APIC
+        end
+    end
+
+    CLI --> IE
+    CLI --> Transfer
+    CLI --> Progress
+    Qt --> IE
+    Qt --> Transfer
+    Qt --> Progress
+
+    U --> CredStore
+    U --> Creds
+
+    US --> APIM
+    U --> APIM
+
+    PMAPIProvider --> APIM
+    EMLProvider --> EML
+    MBOXProvider --> MBOX
+    IMAPProvider --> IMAP
+
+    IE --> US
+    IE --> Transfer
+
+    APIC --> PM
+```