feat(reflog): show a reflog entry's commit in the graph

[patrick-cst1]

Summary

• Adds Show in Graph to the reflog entry context menu: it switches to the graph view and locates the corresponding commit.
• Already loaded: selects the commit and scrolls to it in place (no reload, no message round-trip).
• Not loaded (older than the loaded window, or dangling/unreachable): the extension fetches a pinned slice starting at that commit (git log <hash> --max-count=N, hash as the sole walk start point) and posts it with pinnedHash. The graph selects/scrolls to it and shows a dismissible banner ("Showing commit from reflog" + Clear) to return to the normal graph.
• Unknown/stale/garbage-collected hashes produce a clean error notification instead of failing silently.

Related Issue

Closes #119

Changes

• Extension: GitService.logPinnedCommit() (shared BASE_LOG_FORMAT with log(); hash validation rejects option-like/non-hex), revealCommitInGraph message, MainPanel handler (validates via searchByHash, builds the graph with buildFullGraph, posts logData with the new optional pinnedHash), CommitGraphData.pinnedHash.
• Webview: uiStore.pinnedHash, App.handleShowInGraph/clearPinnedGraph, pinned banner in the graph view, Reflog onShowInGraph prop + menu item, i18n keys in en/ko/zh.
• Tests: new git-service.pinned.test.ts; MainPanel routing/error/pin-clearing; Reflog menu callback; App v1/v2 routing and banner Clear.

Type of Change

• New feature
• Test addition or update

Testing

• New tests fail on unmodified main (20 failing assertions: missing logPinnedCommit, message and handlers) and pass with the change.
• npx tsc --noEmit clean; svelte-check 0 errors / 0 warnings; i18n parity test passes.
• Full suite: 1902 tests, 260 pre-existing Windows/env failures, 0 new failures vs main.

Caveats

• The pinned view replaces the normal graph until Clear, a normal refresh, a full refresh, or a panel/repo reload; the banner is the affordance back.
• The pinned slice is limited to gitGraphPlus.initialCommitCount commits starting at the reflog commit; hasMore is disabled in pinned mode.
• Re-invoking Show in Graph for the commit that is already pinned applies the selection but does not re-run the scroll if the navigation target is unchanged.