fix(connections): cancel and explain foreign-app keychain prompts

[datlechin]

Fixes #1134

Problem

When importing connections from TablePlus / Sequel Ace with passwords:

• macOS prompts for the user's login password once per keychain item because each item's ACL only grants access to its owning app, not TablePro. With ~3 items per TablePlus connection (db / ssh / key), N connections produced up to 3N prompts, which felt like a loop.
• Pressing Esc on a prompt silently treated the read as "no value" and the loop kept going to the next item, prompting again. The loading screen had no Cancel button.
• Esc also let the import "succeed" with no passwords, surprising the issue reporter.

This is by design at the OS level (per-item ACL is intentional to prevent silent cross-app credential siphoning), so the fix is at the UX layer: explain it, give the user a real Cancel, and treat the first denied prompt as "user wants to stop reading credentials".

Changes

• ForeignKeychainReader.readPassword now returns KeychainReadResult { found / notFound / cancelled } instead of String?. Cancellation is no longer indistinguishable from "no value".
• TablePlusImporter and SequelAceImporter carry an abortFlag through their credential reads. On the first .cancelled result, no further keychain reads happen for the remaining connections.
• Both importers also call Task.checkCancellation() between connections so an in-app Cancel button propagates.
• ForeignAppImportResult carries a new credentialsAborted: Bool flag (defaulted to false so DBeaver, which doesn't touch keychain, is unchanged).
• ImportFromAppSheet shows a pre-import NSAlert explaining the per-item prompt behavior and how Always Allow works, only when Include passwords is on. The loading screen has a visible Cancel button that calls Task.cancel().
• ImportFromAppPreviewStep shows an orange banner above the list when credentials were aborted, telling the user to enter remaining passwords manually after import.

Test plan

• Quit TablePlus, install Sequel Ace + TablePlus with several connections. Open TablePro, File > Import from Other App, pick TablePlus with Include passwords ON. Confirm the explainer alert appears with a count.
• Click Continue, then Esc the first macOS keychain prompt. Confirm no further prompts appear and the preview shows the orange "Some passwords were not read" banner.
• Repeat with TablePlus, this time clicking Always Allow on each prompt. Confirm passwords land in the imported connections.
• In the loading screen, click Cancel during a prompt sequence. Confirm the sheet dismisses and no half-imported state remains.
• DBeaver import still works (no keychain involved, no banner, no explainer).
• Source picker with Include passwords OFF skips both the explainer and the prompts entirely.

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