fix: replace eventCache LruCache with ConcurrentHashMap

[barrydeen]

Summary

• Replace eventCache: LruCache<String, NostrEvent>(15000) with ConcurrentHashMap<String, NostrEvent>() to eliminate silent eviction of events
• Add ObjectBox fallback in getEvent() so cross-session lookups resolve from the database when not in memory
• Remove two oversized vector drawables (image2vector.xml, ic_launcher_foreground.xml) with 130KB+ path data strings that caused AAPT2 STRING_TOO_LARGE build failures; switch splash screen icon to splash_icon.xml

Why

LruCache(15000) evicted events under session pressure. Any code calling getEvent(id) after eviction got null — causing orphaned notes in thread view, blank quote embeds, and empty thread views on tap. The events existed in ObjectBox but the cache never fell back to query them.

ConcurrentHashMap has no eviction cap so events stay available for the session lifetime. Memory scales with actual usage (~2k–10k events typical) rather than a fixed 15k ceiling. clearAll() / clearFeed() still wipe it on logout or feed switch.

Test plan

• Scroll feed past a large number of notes, then tap an early note → thread should load correctly
• View a post with a quoted note → embed should resolve
• Navigate to a thread with a parent note seen earlier in the session → parent appears
• Build succeeds without STRING_TOO_LARGE errors
• Splash screen still shows flame icon