Add Civ Lite barbarian camps and events

[YfengJ]

Summary

• Add a deterministic Civ Lite barbarian/event model for camp placement, spawn cadence, raider creation, event selection, and event-choice effects.
• Integrate neutral barbarian camps and hostile raider turns into the game loop so barbarians threaten both player and bot units.
• Add a Frontier Event sidebar panel with deterministic turn events, choice buttons, effect text, event logs, camp tooltips, minimap camp markers, and player rewards for clearing camps.

Validation

• node --test demo/civ-lite/barbarian-model.test.mjs
• node --check demo/civ-lite/barbarian-model.js && node --check demo/civ-lite/game.js && node --check demo/civ-lite/sprites.js
• git diff --check
• Playwright smoke on http://127.0.0.1:4201/demo/civ-lite/: page loaded, canvas pixel sample was non-empty, Frontier Event panel rendered, Turn 4 deterministic event appeared, and choosing an event option updated resources and cleared the panel.

Closes #21

[YfengJ]

CI note: the failing Build Godot Web export job is still stopping at the existing Godot Web preset configuration error: Cannot export project with preset Web due to configuration errors. This PR only changes demo/civ-lite JS/HTML/CSS plus a local barbarian-model test. Local validation passed: node --test demo/civ-lite/barbarian-model.test.mjs; node --check demo/civ-lite/barbarian-model.js; node --check demo/civ-lite/game.js; node --check demo/civ-lite/sprites.js; git diff --check; and a Playwright smoke check for page load, non-empty canvas, Frontier Event rendering, deterministic Turn 4 event display, and event-choice resource updates. SonarCloud Quality Gate passes with 0 new issues on latest head d74db63.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud