add workspace directory feature with shell quoting and tests

[rnunley-nmg]

• Add directory field to workspaces for setting default working directory
• Terminals cd to workspace directory on change; preview blocks navigate to it
• Add folder picker dialog (Electron showOpenDialog)
• Add shellQuote utility for safe shell command construction
• Add path validation to reject null bytes in directory paths
• Debounce directory updates to prevent rapid API calls
• Expose WAVETERM_WORKSPACE_DIR environment variable to shells
• Add frontend tests for shellQuote function
• Add Go tests for workspace directory validation