fix: clean account caches on deletion

[FromSi]

What?

This PR updates contact usage tracking and cleans account-owned cache data when an account is deleted.

Commit 1: contact usage per account

Contacts now store usage metadata per account instead of a single global last_used/use_count pair.

Old contact JSON:

{
  "name": "Alice",
  "email": "alice@example.com",
- "last_used": "2026-05-06T19:29:28.502623767+05:00",
- "use_count": 1
}

New contact JSON:

{
  "name": "Alice",
  "email": "alice@example.com",
+ "usage_by_account": {
+   "account-id": {
+     "last_used": "2026-05-06T19:29:28.502623767+05:00",
+     "use_count": 1
+   }
+ }
}

This includes migration support for legacy contacts, account-aware contact search, account-aware contact saving, macOS contact sync updates, and small generic collection helpers.

Commit 2: account cache cleanup

Adds a single config.CleanupAccountCache(accountID) method that removes data associated with a deleted account from:

• email_cache.json
• folder_cache.json
• folder_emails/*.json
• email_bodies/*.json
• contacts.json
• drafts.json

The account deletion flow now calls this cleanup after RemoveAccount.

Why?

Closes: #519

Fixes stale account data being left behind after deleting an account.

Previously, deleting an account removed it from config and deleted credentials from the keyring, but cached emails, cached bodies, drafts, folder cache entries, and contact usage stayed on disk. Over time this left orphaned data in ~/.cache/matcha.

The contact schema change makes it possible to remove only the deleted account’s contact association while preserving shared contacts still used by other accounts.

[andrinoff]

big PR, i'll check it out later, thx @FromSi

[andrinoff]

I'll review these changes way later, very busy at the moment, won't release the next version without this, so no worries

[andrinoff]

lgtm