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

[floatpanebot]

This PR exceeds the recommended size of 1000 lines. Please make sure you are NOT addressing multiple issues with one PR. Note this PR might be rejected due to its size.