feature: add AI-powered Git diff analysis for working tree and commit range comparison

[YuHaoLiaoTony]

This PR adds an AI-powered Git diff analysis feature that lets users get a natural-language summary of their changes using their configured OpenAI-compatible AI service.

What's new

• Working tree analysis: Analyze staged + unstaged changes directly from the commit message toolbar
• Commit range analysis: Select two commits in History and analyze the diff between them, accessible via right-click context menu or the Revision Compare toolbar button
• Locale-aware AI output: Automatically maps the app's UI locale (16 languages) into the AI prompt, so the summary is generated in the user's language
• Large diff protection: Automatically truncates diffs exceeding 100K characters or 3K lines; individual files over 20K chars are skipped
• Binary/LFS skipping: Binary files and LFS pointers are automatically detected and excluded from the analysis payload
• Markdown rendering: AI output is rendered with proper heading, code block, and inline code formatting using a new MarkdownResultView control
• Model selection: The analysis dialog includes a model ComboBox matching the existing AI commit message UX, disabled during analysis to prevent mid-request model switching

Architecture

All new code follows the existing MVVM and AI module patterns:

• AI/ layer: AIDiffContextData (data model), AIDiffContextBuilder (git command execution with SHA validation), DiffPrompts (prompt templates + locale mapping), DiffAgent (thin API call wrapper)
• ViewModels/AIDiffAnalysis.cs: state management, cancellation, retry, error mapping
• Views/AIDiffAnalysis.axaml + MarkdownResultView.axaml: popup dialog with Markdown rendering

Existing file changes

Minimal modifications to 7 existing files:

• RevisionCompare.cs: exposed GetSHA() and Repository property (internal → public)
• CommitMessageToolBox.axaml + .cs: one toolbar button + handler
• Histories.axaml.cs: context menu item for 2-commit selection + helper
• RevisionCompare.axaml + .cs: one title bar button + handler
• AIAssistant.axaml.cs: enabled WordWrap on AIResponseView
• en_US.axaml + 14 locale files: 18 new localization keys each

Verification

• dotnet build succeeds (0 errors, 0 warnings)
• No AI service configured → user notification shown
• No changes to analyze → user notification shown
• Single AI service → dialog opens directly
• Multiple AI services → service selection menu shown
• Binary/LFS files → skipped with mention in output
• Large diffs → truncated to stat-only mode