Split the extractable parts out of the mail store

store/mail.ts was 1,463 lines. It is now a directory, so `@/store/mail`
resolves to index.ts and none of the 36 modules importing `useMail`
changes a line:

  mail/props.ts      72   MAILBOX_PROPS, LIST_PROPS, FULL_PROPS, BODY_PROPS
  mail/types.ts     125   ListQuery, ListState, MailState, DEFAULT_SORT
  mail/mailboxes.ts  28   mailboxIcon, ROLE_ORDER
  mail/index.ts   1,266   the store, and everything bound to it

Everything exported before is still exported from index.ts, so this is
file layout and nothing else. No behavior change, no call-site change.

WHAT THIS DOES NOT DO, and why. index.ts is still 1,266 lines because
947 of them are one `create<MailState>((set, get) => ({ ... }))`. Cutting
that up means Zustand slices -- splitting the state object itself and
recombining it -- which is a change to how the store is built rather than
to where its text lives, in the part of the app that every screen leans
on. That deserves its own PR and its own argument, not a quiet ride along
with a file move.

Three things had to stay behind and are worth knowing about, because the
obvious boundary is wrong in each case:

  - `listKey` sits among the type declarations but is a function the
    store calls, not a type.
  - `ensureFolderPath`, `folderRefs` and `followFolders` read like folder
    helpers and look like they belong beside mailboxIcon, but they close
    over `useMail`. Moving them makes mailboxes.ts import index.ts, which
    imports mailboxes.ts.
  - the sieve import inside index.ts is `await import(...)`, not a static
    one, so rewriting import paths by their `from` clause misses it.

web/src/store/mail/props.ts

@@ -0,0 +1,72 @@
+import { SPAM_HEADER_PROPS } from "@/lib/spamScore";
+
+
+/*
+ * Named explicitly so `shareWith` comes back, which it does not otherwise --
+ * see the note on CALENDAR_PROPS and the KNOWN-ISSUES entry. Mailboxes were the
+ * third and last store fetching everything by asking for nothing.
+ *
+ * It matters here for one narrow but real case. Sharing a mail folder is
+ * withdrawn because Stalwart stores the share and never delivers it, and the
+ * only way left to clear one already made is the "Stop sharing" entry, which
+ * appears only when a folder looks shared. Without this it never looked shared,
+ * so the escape hatch for the exact situation it was built for was invisible.
+ */
+export const MAILBOX_PROPS = [
+  "id",
+  "name",
+  "parentId",
+  "role",
+  "sortOrder",
+  "totalEmails",
+  "unreadEmails",
+  "totalThreads",
+  "unreadThreads",
+  "myRights",
+  "isSubscribed",
+  "shareWith",
+];
+
+export const LIST_PROPS = [
+  "id",
+  "blobId",
+  "threadId",
+  "mailboxIds",
+  "keywords",
+  "hasAttachment",
+  "from",
+  "to",
+  "subject",
+  "receivedAt",
+  "sentAt",
+  "size",
+  "preview",
+];
+
+export const FULL_PROPS = [
+  ...LIST_PROPS,
+  "messageId",
+  "inReplyTo",
+  "references",
+  "sender",
+  "cc",
+  "bcc",
+  "replyTo",
+  "bodyStructure",
+  "bodyValues",
+  "textBody",
+  "htmlBody",
+  "attachments",
+  "header:List-Unsubscribe:asText",
+  "header:List-Unsubscribe-Post:asText",
+  "header:List-Id:asText",
+  "header:Disposition-Notification-To:asAddresses",
+  "header:X-Priority:asText",
+  "header:Importance:asText",
+  "header:Auto-Submitted:asText",
+  "header:Precedence:asText",
+  "header:Authentication-Results:asText",
+  ...SPAM_HEADER_PROPS,
+];
+
+export const BODY_PROPS = ["partId", "blobId", "size", "name", "type", "charset", "disposition", "cid", "language", "location", "subParts", "headers"];