[feat](trx-frontend-http): split app.js into ES modules with lazy loading

Extract map-core.js (3,483 lines) and screenshot.js (261 lines) from
the monolithic app.js, reducing it by ~30% (11,967 → 8,427 lines).

Modules communicate via a window.trx shared namespace with getter/setter-
backed state proxying. Map and statistics code lazy-loads on first tab
activation; screenshot code lazy-loads on first "S" keypress. All cross-
module calls use optional chaining for safe access before modules load.

Adds Rust infrastructure (include_str, gz_cache, Actix routes) for
serving the new JS assets.

https://claude.ai/code/session_01HgW8UpscRRA3CgSLqQDzdp
Signed-off-by: Claude <noreply@anthropic.com>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/index.html

@@ -1563,7 +1563,8 @@
 (function() {
   var pluginScripts = {
     'digital-modes': ['/ft8.js', '/ft4.js', '/ft2.js', '/wspr.js', '/cw.js', '/background-decode.js'],
-    'map': ['/leaflet-ais-tracksymbol.js', '/ais.js', '/vdes.js', '/aprs.js', '/hf-aprs.js', '/sat.js', '/sat-scheduler.js'],
+    'map': ['/map-core.js', '/leaflet-ais-tracksymbol.js', '/ais.js', '/vdes.js', '/aprs.js', '/hf-aprs.js', '/sat.js', '/sat-scheduler.js'],
+    'statistics': ['/map-core.js'],
     'bookmarks': ['/bookmarks.js'],
     'recorder': ['/scheduler.js'],
     'settings': ['/vchan.js']