feat: in-memory LRU email body cache

[mavonx]

What?

Replaces the disk-only email body cache with an in-memory LRU cache backed by write-through disk persistence.

• Single global LRU instance shared across all folders via sync.Once singleton.
• Fast Get() and Put() operations — O(1) via hashmap + doubly-linked list.
• Write-through: every Get() and Put() immediately persists to disk, so no data is lost on crash.
• Eviction removes least-recently-used entries from both memory and disk atomically.
• On the first GetLRUInstance() call, LoadFromDisk() restores LRU order using LastAccessedAt timestamps sorted oldest-first, so the most recently accessed email ends up at the front.

Why?

The previous disk-only approach had two bottlenecks: GetCachedEmailBody() read the entire folder JSON file on every call, and pruneEmailBodyCacheSize() loaded all folder files on every SaveEmailBody() call. With multiple accounts and many folders, this becomes expensive.

Related #1171