Add stickyHeader setting to fix graph header while scrolling

[plitzenberger]

Summary

Adds a new extension setting git-graph.stickyHeader (default true) that keeps the Graph View control bar and column headers fixed at the top while scrolling.

This implements the long-requested sticky header behavior (upstream #132 / #394):

• New setting wired through package.json, Config, GitGraphViewConfig, and the webview initial state
• When enabled, #controls and #tableColHeaders use position: sticky
• Table header top offset tracks control-bar height (including multi-line overflow)
• Scroll shadow is positioned beneath the sticky header stack
• Context menu uses fixed positioning to avoid overlapping sticky headers

Disable with:

"git-graph.stickyHeader": false

Test plan

• Unit tests for Config.stickyHeader
• HTML generation tests for sticky class on/off
• Related test suites pass (config.test, gitGraphView.test — 439 tests)
• Lint passes
• Full npm run compile succeeds
• Manually verify sticky header while scrolling in the Graph View
• Manually verify header remains sticky when the control bar wraps to multiple lines
• Manually verify scroll shadow appears under column headers when scrolled
• Manually verify dropdowns and context menus still render correctly with sticky enabled
• Confirm "git-graph.stickyHeader": false restores previous scroll-away behavior