feat: use Result<T> in email settings save path instead of throwing

[andregoepel]

Closes #141, scoped down from its literal wording after re-reading the actual code:

Fixed — the real convention violation:

• IEmailSettingsStore.SaveAsync / MartenEmailSettingsStore.SaveAsync now return Result instead of throwing InvalidOperationException for the "no password on first save" case — that's an ordinary validation outcome, not an exceptional condition.
• EmailSettingsPage.razor's OnValidSubmit branches on the result instead of catch (Exception ex), and shows result.Error (our own controlled validation message) instead of ex.Message.
• Added a new test (Submit_StoreReturnsFailure_ShowsErrorNotificationWithResultMessage) covering the failure-notification path, which had no test coverage before.

Deliberately NOT changed, with reasoning:

• EmailSettingsPage.razor's OnSendTest (catch (Exception ex) around the test-email send) — this wraps genuine external SMTP I/O with many real failure modes (network, auth, DNS), not an expected validation condition. Converting it to Result<T> wouldn't fix anything; the ex.Message shown there is also gated behind [Authorize(Roles = "Administrator")], so it's diagnostic info for the person testing their own mail settings, not a leak to an untrusted party.
• Setup.razor's catch (Exception ex) (line ~232) — on closer read this already logs properly (Logger.LogError(ex, "Critical error during setup")) and shows a safe, generic message ("Setup could not be completed. Please try again.") — it never leaks ex.Message. It's a legitimate top-level safety net around a multi-step admin-bootstrap flow whose individual steps already branch on typed results (IdentityResult, SignInResult). This isn't the "exceptions for flow control" anti-pattern; converting it would add complexity without fixing anything.

Verification: dotnet csharpier format . clean, dotnet build 0 errors, dotnet test --filter "FullyQualifiedName!~E2ETests" 118/118 passed (117 existing + 1 new), dotnet list package --vulnerable --include-transitive clean.

Closes #141

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