style: Multiple low-severity style violations across the codebase

[jaybarden1]

Aggregated low-severity style violations

These are individually small but collectively degrade code consistency and will generate warnings (→ errors with TreatWarningsAsErrors=true).

1. XML <param name="ct"> mismatch — CS1572 compile error

IAccountRepository, ISyncRepository, IFileClassificationRuleRepository, WorkspaceViewModel.LoadPersistedAccountsAsync: Every method documents the cancellation parameter as <param name="ct"> but the actual signatures use cancellationToken. With TreatWarningsAsErrors=true this is a compile error (CS1572).

Fix: Replace all <param name="ct"> with <param name="cancellationToken"> across those files.

2. Missing sealed on classes that should not be inherited

• WorkspaceViewModel → public sealed class WorkspaceViewModel
• AccountViewModel → public sealed class AccountViewModel
• FolderNode → public sealed class FolderNode
• Program → internal static sealed class Program

3. return not preceded by blank line in AuthService catch blocks

Three catch blocks (lines ~38, 43, 68) have a return statement on the line immediately following the log call, with no blank line between. Per convention, every return after a code block must be preceded by a blank line.

Fix (for each): add a blank line before the return.

4. BuildAvaloniaApp() missing XML <summary> and <returns>

Program.cs:14: public static AppBuilder BuildAvaloniaApp() is public with no XML docs.

5. ConfigureAwait(false) missing on multiple awaits

• AccountOnboardingService.cs — final await on the MatchAsync chain
• AddAccountWizardViewModel.cs — three service-layer await calls
• App.axaml.cs — four await calls in InitialiseAsync and ApplyDatabaseMigrationsAsync
• WorkspaceViewModel.cs — await _accountRepository.GetAllAsync(...)

Add .ConfigureAwait(false) on each.

6. XML summary comments that restate the code (banned "what" comments)

• Auth/AccountId.cs: factory method summary speculates about future validation. Replace with a terse contract statement.
• Auth/AccountProfile.cs: same pattern.
• Auth/AuthError.cs/Auth/AuthResult.cs: check for similar.

7. AccountProfileFactory class missing <summary>

Auth/AccountProfile.cs:8: the factory class itself has no XML <summary>.

Rule reference

@.claude/rules/c-sharp-code-style.md — XML Documentation, Classes and Methods, naming