Extract shared display formatting into Rust

[kwsantiago]

Context

Both keep-desktop and keep-android independently implement display formatting logic that should live in Rust once and be consumed by both platforms. This was identified by comparing against the RMP Architecture Bible patterns, which calls this the "duplicated formatting logic" anti-pattern.

Current duplication

Formatting	Desktop (Rust)	Android (Kotlin)
npub truncation	screen/mod.rs::truncate_npub()	NostrFormatting.kt
Timestamps	screen/mod.rs::format_timestamp()	Various inline SimpleDateFormat calls
Share display names	screen/shares.rs::ShareEntry::from_stored()	ShareCards.kt
Hex truncation	screen/signing_audit.rs::truncate_hex()	Inline in composables

Proposed changes

1. Add display formatting functions to keep-core (or a shared module both platforms already depend on):

   • truncate_npub(npub: &str, prefix: usize, suffix: usize) -> String
   • format_timestamp(epoch_secs: u64) -> String (relative: "2 min ago", "yesterday", etc.)
   • truncate_hex(hex: &str, len: usize) -> String
   • format_share_display_name(name: &str, group_pubkey: &[u8]) -> String

2. Update keep-desktop screens to call shared functions instead of local copies

3. Expose via UniFFI in keep-mobile so keep-android can drop its Kotlin implementations

Benefit

Single source of truth for all user-visible string formatting. No more divergence between platforms.