feat(bar): configurable height + optional gradient

[ramakrishnajoshi]

Adds configurable top bar appearance:

• bar.height: set bar thickness (clamped 1–10)
• bar.gradient.enabled: optional vertical fade (top opaque → bottom transparent)
• bar.gradient.fadePercent: portion of height used for fade (1–100)

Behavior:

• Backward compatible: missing keys fall back to old look (3pt, no gradient)
• Fresh installs: generated config includes new defaults
• Live reload on config edits preserved

Files:

• IsThereNetApp.swift: implements BarConfig, gradient, and height logic
• IsThereNet/config.json: template updated with defaults
• README.md: docs for new options

Validation notes:

• Gradient auto-disables when height < 2
• Height clamped to fit existing 20pt window chrome