Add environment variables configuration to OpenCode instances

- Add environment variables editor in advanced settings
- Store environment variables in global preferences
- Pass environment variables to OpenCode processes when spawning
- Display environment variables in instance information and server logs
- Fix system PATH binary handling for 'opencode' command
- Show detailed environment variable values in startup logs
- Save and restore last used binary across app restarts
- Add system PATH binary to recent binaries list when used

Features:
- Environment variables editor with add/remove functionality
- Persistent storage of environment variables across sessions
- Real-time display of active environment variables in logs
- Instance information shows configured environment variables
- Proper handling of system PATH vs custom binary paths
- Last used binary persistence and automatic restoration

src/lib/storage.ts

@@ -51,6 +51,7 @@ export class FileStorage {
       return {
         preferences: {
           showThinkingBlocks: false,
+          environmentVariables: {},
         },
         recentFolders: [],
         opencodeBinaries: [],