fix(connections): stop Export Options sheet from resizing and save via fileExporter

[datlechin]

What

Reworks the Welcome window's Export Options sheet (right-click a connection > Export) so it behaves like a native macOS dialog.

Why

Three defects, all from the view fighting macOS conventions:

1. Resized on toggle. A grouped Form that conditionally added/removed Sections, with a fixed width but no height, made the sheet jump when "Include Credentials" was turned on.
2. Over-chromed. Form(.formStyle(.grouped)) inside hand-built title/divider/footer chrome rendered a card-in-a-card, inconsistent with the sibling import sheet.
3. Fragile save flow. Export dismissed the sheet, slept 200ms, re-acquired NSApp.keyWindow, then opened an NSSavePanel by hand, hand-rolling what .fileExporter already does (and what the import side already uses via .fileImporter).

What changed

• Plain VStack/Grid layout in one fixed frame(width: 440, height: 300). Passphrase fields reveal in pre-sized space; the validation message sits in a reserved slot, so the window never resizes.
• Save now uses SwiftUI .fileExporter with a new ConnectionExportDocument: FileDocument. Removed the dismiss + Task.sleep(200ms) + NSApp.keyWindow workaround.
• Passphrase rules extracted into a pure, testable ConnectionExportPassphraseState enum; Export stays disabled until valid, with inline warnings.
• ConnectionExportService gained Data-returning exportData / exportEncryptedData; the existing URL writers now call them (signatures unchanged).

Pro gating and security (passphrase zeroed once the ciphertext is built; the document holds only encrypted bytes) are preserved.

Tests

New ConnectionExportDataTests: plaintext and encrypted round-trips (right and wrong passphrase) plus validator-state cases. This is the first coverage of the export pipeline.

The system save panel is a system dialog and is not UI-automatable, so the save step itself is not covered by a UI test.

Notes

SwiftLint --strict passes on the changed files. The numbered steps in docs/features/connection-sharing.mdx stay accurate (the flow is unchanged), so no docs text change.