Add mailbox size and attachments count to stats overview

[bkmdev]

Motivation

• Provide mailbox storage metrics requested in issue Add metrics about attachments and email storage size devmount/third-stats#213 by using Thunderbird MessageHeader attributes to surface total mailbox size and number of messages with attachments in the overview.

Description

• src/Stats.vue: imported formatBytes, extended initData().numbers with totalSize and withAttachments, and accumulated m.size and m.hasAttachments in analyzeMessage while processing messages.
• src/Stats.vue: included the new metrics in the multi-account sum aggregation logic so the "All Accounts" view reports correct totals.
• src/Stats.vue: added computed properties totalMailSize and mailsWithAttachments and rendered a new featured stats card showing human-readable total mailbox size and the count of mails with attachments.
• public/_locales/en/messages.json: added English locale strings stats.totalMailSize and stats.mailsWithAttachments for the new UI labels.

Testing

• Ran the production build with npm run build, which completed successfully without errors.

---

Codex Task