Add encrypted Mail Profile foundation - #448

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In
`@apps/unwired-mail/unwired-mail/Services/ProductSync/MailboxConnectionSyncModels.swift`:
- Around line 301-319: The reconciliation logic around activeIds and
retainedAssignments must preserve assignments for connections absent from the
current payload, removing them only when an explicit connection-removal
tombstone exists; continue assigning default profiles to newly active,
unassigned connections. In
apps/unwired-mail/unwired-mail/Services/ProductSync/MailboxConnectionSyncModels.swift#L301-L319,
update the filtering logic accordingly. In
docs/adr/0048-separate-encrypted-mail-profile-ownership.md#L17-L17, document
this retention rule in the Decision and the resulting ownership consequence in
Consequences.
- Around line 285-300: Update the profile-repair logic before the guard in the
validation flow so a non-nil defaultProfileId that is absent from profiles
re-adds defaultProfile, resets defaultProfileId to defaultProfile.id, and marks
changed. Keep the repair deterministic and idempotent, then allow the existing
guard to validate the repaired profile set.

In
`@apps/unwired-mail/unwired-mail/Services/ProductSync/MailboxConnectionSyncService.swift`:
- Around line 568-574: Update the profile-name validation in createProfile and
validateProfilePayload to reject duplicate names and names exceeding 40
characters with a dedicated MailProfileSyncError case, rather than
concurrentModification or generic invalidProfileState. Ensure the new error
communicates invalid or duplicate input and is used consistently for both
validation paths.
- Around line 774-786: Update MailProfileFieldValue.set(_:for:) to return Bool,
returning true only when the field and value cases match and are assigned, and
false for mismatches. In resolveProfileConflict, check this result before
removing the conflict or persisting the profile, and reject the resolution when
it is false so mismatched synchronized payloads cannot be silently discarded.
- Around line 681-694: Update updateProfiles so
Self.validateProfilePayload(payload) runs only when mutation returns true and
the payload will be written; allow the changed == false read path to return
.acceptAuthoritative without validating legacy or mixed-version values. Preserve
sorting and resolvedPayload assignment for both paths.

In `@apps/unwired-mail/unwired-mailTests/MailboxConnectionSyncServiceTests.swift`:
- Around line 1127-1149: Add tests alongside
testDefaultProfileMigrationIsIdempotentAndClaimsConnectionsAddedByOlderClients
covering three gaps: retain a non-default Profile assignment when its connection
is removed from and later restored to the payload; accept and return a seeded
mail-profiles-primary payload containing a Profile name longer than 40
characters; and verify a Profile operation throws
MailProfileSyncError.missingProductSyncKeyMaterial when key material is absent
while another product-sync payload exists. Reuse the existing services, payload,
and error-test setup patterns.

In `@CONTEXT.md`:
- Around line 395-406: Add relationship bullets in the Relationships section
covering that each Mailbox Connection belongs to exactly one Mail Profile,
Profile-scoped queries require an explicit Mail Profile, and provider
credentials remain device-local and outside Profile Record Scope. Align the
terminology with Mail Profile, Mailbox Connection, and Profile Record Scope as
defined in the diff and ADR 0048.

In `@docs/adr/0048-separate-encrypted-mail-profile-ownership.md`:
- Line 17: Update the ADR’s reconciliation decision, including the text around
the default-profile behavior, to explicitly document what happens when an absent
connection later returns: state that its prior Profile ownership is removed and
it is reassigned to the Default Profile, or document the retention rule that
prevents this. Include the resulting user-visible consequence and any relevant
privacy or security boundary impact.