v0.0.41: FC-P6-T2 — read receipts when messages are visible

- ChatLine gains sender_fp field for tracking who sent each message
- App gains read_receipts_sent HashSet to avoid duplicate receipts
- After each draw(), visible received messages get a Read receipt sent
- Only fires once per message_id, skips system/self messages
- Sender sees blue ✓✓ (existing display logic already handles Read)
- All ChatLine literals across 6 files updated with sender_fp field

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

warzone/crates/warzone-server/src/routes/web.rs

@@ -50,7 +50,7 @@ async fn pwa_manifest() -> impl IntoResponse {
 
 async fn service_worker() -> impl IntoResponse {
     ([(header::CONTENT_TYPE, "application/javascript")], r##"
-const CACHE = 'wz-v22';
+const CACHE = 'wz-v23';
 const SHELL = ['/', '/wasm/warzone_wasm.js', '/wasm/warzone_wasm_bg.wasm', '/icon.svg', '/manifest.json'];
 
 self.addEventListener('install', e => {
@@ -287,7 +287,7 @@ let pollTimer = null;
 let ws = null;  // WebSocket connection
 let wasmReady = false;
 
-const VERSION = '0.0.40';
+const VERSION = '0.0.41';
 let DEBUG = true;  // toggle with /debug command
 
 // ── Receipt tracking ──