Add pause menu with Trains/Bag/Save/Options

[githumps]

Summary

Implement the main pause menu accessible during overworld exploration, providing access to core management screens.

Why

Players need to check their party, use items, and save the game during exploration. Currently no unified menu system exists.

Acceptance Criteria

• Pressing Start (or menu button on mobile) opens pause menu overlay
• Four options: TRAINS, BAG, SAVE, OPTIONS
• TRAINS: View party stats, moves, HP - no actions needed yet
• BAG: Open item list (basic view for MVP)
• SAVE: Trigger manual save with confirmation
• OPTIONS: Placeholder or basic settings (text speed, audio)
• Menu navigable via keyboard and mobile touch controls
• Can exit menu with B/Back button

Notes

• Related to advanced menu system ([conducted] Build unified menu overlays (Bag, Trains, Save, Options) with mobile support #37)
• Bag integration depends on item system (Implement item system with bag, inventory, and item effects #51)
• Mobile controls already exist per README.md

[githumps]

Additional Bug Found During Testing

Escape key restarts the whole game instead of opening the pause menu.

Expected Behavior

• Press Escape → Open pause menu (Trains/Bag/Traindex/Save/Options)
• Press Escape again → Close menu, return to game

Current Behavior

• Press Escape → Game restarts from title screen

Priority Upgrade

Upgrading to P0-critical - blocks all menu access

Files to Check

• js/input.js (Escape key handler)
• js/ui.js (pause menu implementation)

[githumps]

Reopening: Pause menu was marked done but is not actually implemented in code. Starting PR 1.1 implementation per M1 MVP plan.