fix(rebase): reconcile viewer blob routing and dedupe settings after main rebase

Rebasing feat/unified-mailbox onto main hit deep, divergent conflicts in the
mail-view/settings area (main added its own All-Mail + RTL refactor + a
username-keyed #507 identity impl). Post-rebase reconciliation:

- re-apply the cross-account blob routing to the message viewer (inline images,
  drag-out, TNEF, embedded messages, thumbnails, bundle download) on main's
  restructured file — every fetch goes through blobClient/blobAccountId derived
  from the message's source account;
- drop the duplicate `preferredIdentityIds` declaration that both main
  (username-keyed) and the branch (accountId-keyed) introduced — the branch's
  account-scoped map is kept, matching the resolved modal/store logic.

tsc + eslint clean; unified-mailbox unit tests pass (settings-store all-mail /
preferred-identity, unified-mailbox-cross, jmap-client-resilience, migrate-policy).

stores/settings-store.ts

@@ -178,12 +178,6 @@ interface SettingsState {
   requestReadReceiptDefault: boolean; // Pre-check "request read receipt" in the composer
   readReceiptResponse: ReadReceiptResponse; // How to respond to incoming read-receipt requests
 
-  // Identities
-  // Per-account default ("preferred primary") sender identity, keyed by
-  // username (the same key settings sync uses). A JMAP identity id is only
-  // meaningful within its own account, so this must be account-scoped. Synced
-  // so the choice survives a new browser / cleared site data (#507).
-  preferredIdentityIds: Record<string, string | null>;
 
   // Privacy & Security
   sessionTimeout: number; // minutes (0 = never)
@@ -406,8 +400,6 @@ const DEFAULT_SETTINGS = {
   requestReadReceiptDefault: false,
   readReceiptResponse: 'ask' as ReadReceiptResponse,
 
-  // Identities
-  preferredIdentityIds: {} as Record<string, string | null>,
 
   // Privacy & Security
   sessionTimeout: 0, // Never
@@ -619,7 +611,6 @@ export const useSettingsStore = create<SettingsState>()(
           signatureSeparatorEnabled: state.signatureSeparatorEnabled,
           requestReadReceiptDefault: state.requestReadReceiptDefault,
           readReceiptResponse: state.readReceiptResponse,
-          preferredIdentityIds: state.preferredIdentityIds,
           sessionTimeout: state.sessionTimeout,
           emailNotificationsEnabled: state.emailNotificationsEnabled,
           emailNotificationSound: state.emailNotificationSound,