fix: prevent modifier-key combos (Ctrl+C, Ctrl+S) from triggering single-key shortcuts

[YizukiAme]

Summary

Fixes #358 — Ctrl+C (copy), Ctrl+S (save), and other modifier-key combinations were incorrectly intercepted by the global keyboard shortcut handler in Stage, toggling UI panels instead of performing their native browser actions.

Problem

The onKeyDown handler in components/stage.tsx matches single-letter keys (c, s, m) without checking for modifier keys. When a user presses Ctrl+C:

1. event.key → 'c' ✅ matches the case 'c' branch
2. event.ctrlKey → true ⚠️ not checked
3. event.preventDefault() blocks the native copy action
4. setChatAreaCollapsed(!chatAreaCollapsed) toggles the chat sidebar

Combination	Expected	Actual (before fix)
Ctrl+C	Copy	Toggles chat sidebar
Ctrl+S	Save	Toggles scene sidebar
Ctrl+M	Browser shortcut	Toggles TTS mute
⌘+C / ⌘+S (macOS)	Copy / Save	Same issue via metaKey

Fix

Add a single modifier-key guard at the top of the onKeyDown handler:

if (event.ctrlKey || event.metaKey || event.altKey) return;

This lets all modifier-key combinations pass through to the browser while preserving bare-key shortcuts (c, s, m, arrows, space, escape).

Risk Assessment

Minimal. No existing shortcut in this handler is designed to work with Ctrl/Meta/Alt. The guard only affects key events that were never intended to be captured.