feat(graph): optional auto-load of further history

[patrick-cst1]

Summary

• Adds gitGraphPlus.autoLoadHistory (default false). When enabled, scrolling near the bottom loads the next chunk automatically (same request as the "Load more commits" button); duplicate in-flight requests are guarded. Disabled behaviour is unchanged.

Related Issue

Closes #61

Changes

• Setting + readAutoLoadHistory(), setAutoLoadHistory message, store flag, App handler; isNearBottom() helper; CommitGraph triggers the shared loadMore().
• Tests: config reader, App message handling, auto-load helper, CommitGraph scroll trigger/in-flight guard.

Type of Change

• New feature
• Test addition or update

Testing

• New tests fail on unmodified main and pass with the change; svelte-check 0 errors.- npx tsc --noEmit clean.
• Full suite on Windows: 260 pre-existing failures (POSIX-only integration helper + env), 0 new failures vs main; focused tests pass.
• The repo's integration suite only runs on Linux (its helper uses POSIX quoting), CI covers those.

Caveats

• No polling and no new listeners: it hooks the existing scroll handler.