feat: add option to prompt for database password on every connection

[datlechin]

Summary

Closes #534

• Adds a toggle in the connection form: "Ask for password on every connection"
• When enabled, passwords are never saved to Keychain — user is prompted via native NSAlert modal each time they connect
• Prompted password is cached in-memory for the session lifetime (auto-reconnect uses cache silently, app relaunch re-prompts)
• Works for both database passwords and API tokens (BigQuery, D1, DynamoDB)

Implementation

File	Change
DatabaseConnection.swift	promptForPassword computed property via additionalFields
ConnectionSession.swift	cachedPassword: String? in-memory session cache
PasswordPromptHelper.swift	Native NSAlert + NSSecureTextField password prompt
PasswordPromptToggle.swift	Extracted toggle + conditional SecureField view
DatabaseDriver.swift	passwordOverride param on createDriver/resolvePassword
DatabaseManager.swift	Prompt in connectToSession, cache in reconnect paths
ConnectionFormView.swift	Toggle UI, save/load/test/validation
ConnectionStorage.swift	Skip password copy in duplicateConnection
ConnectionSSLView.swift	Fix pre-existing missing UniformTypeIdentifiers import

Edge cases handled

• Save → Prompt switch: deletes existing Keychain password
• Auto-reconnect: uses session-cached password silently
• App relaunch: re-prompts (cache not persisted)
• Cancel prompt: clean cancellation, no stale session
• usePgpass mutual exclusion: toggling one disables the other
• Duplicate connection: flag copied, password not copied

Test plan

• Create new connection → toggle ON → save → connect → verify prompt appears, no Keychain entry
• Edit existing connection → toggle ON → save → verify old Keychain password deleted
• Toggle OFF → enter password → save → verify Keychain password saved again
• Connect with prompt → kill DB → wait for auto-reconnect → verify no re-prompt
• Quit app → relaunch → verify prompt appears on session restore
• Duplicate a prompt-for-password connection → verify duplicate also prompts
• Cancel prompt → verify clean state (no stuck connecting indicator)
• Toggle usePgpass ON → verify promptForPassword disables, and vice versa
• API-only connection (e.g. BigQuery) → verify toggle says "Ask for API token"