refactor(ios): extract connection form logic into ConnectionFormViewModel

[datlechin]

Summary

P1 #5 step 2 of 3: lift business logic out of ConnectionFormView (652 LOC) into a new ConnectionFormViewModel. The View drops to 392 LOC and now owns only UI state (file-picker flags, new-database alert flag, haptic triggers); the ViewModel owns form fields, validation, async operations, and persistence.

What moves into the ViewModel:

• All form state: name, type, host, port, username, password, database, sslEnabled, groupId, tagId, safeModeLevel, plus the 11 SSH fields and selectedFileURL / newDatabaseName
• Async operations: loadStoredCredentials(secureStore:) (replaces inline keychain reads in .task), testConnection(appState:secureStore:), save(appState:secureStore:) -> DatabaseConnection? returning the connection on success or nil with credentialError set
• File picker handlers: handleSQLiteFilePicker(_:), handleSSHKeyFilePicker(_:), createNewDatabase(), clearSelectedFile()
• Helpers: buildConnection(), copyToDocuments(_:), updateDefaultPort(), canSave, isEditing
• Type-change reactivity uses a didSet on type to reset port and SQLite file selection, replacing the inline .onChange(of: type)
• The KeyInputMode enum and the TestResult struct move to the VM where they belong

What stays in the View:

• 5 @State: viewModel, activeFilePicker, pendingFilePicker, showNewDatabaseAlert, hapticSuccess, hapticError
• The dual activeFilePicker + pendingFilePicker is preserved on purpose: .fileImporter resets activeFilePicker via the dismissal binding before the result closure fires, so the result handler reads pendingFilePicker instead
• Form layout, sections, picker rows, and toolbar
• The keychain warning alert binds to viewModel.credentialError != nil via a derived Binding<Bool> (no parallel @State flag)

Test plan

• Open "New Connection": defaults populate (MySQL, 127.0.0.1:3306). Switch types: port updates (5432 for Postgres, 6379 for Redis, empty for SQLite), SQLite file selection clears
• SQLite: Open Database File copies into Documents and prefills name; Create New Database prompts and assigns .db extension
• SSH tunnel: toggle on, fill server, switch auth modes (password / private key), import key file (file persists into Documents prefixed with ssh_), paste key content
• Test Connection (success): green checkmark + "Connection successful", success haptic. Failure: error message + recovery hint, error haptic
• Save with valid credentials: parent receives the new DatabaseConnection via onSave, sheet dismisses
• Save with keychain failure (force by toggling Keychain access): "Keychain Warning" alert appears, save aborts, sheet stays open
• Edit existing connection: loadStoredCredentials hydrates password / SSH password / passphrase from Keychain on .task