feat: add git panel source control workflows

[jouwdan]

Summary

• Renames the Diff surface to a full Git panel with staged/unstaged sections, file tree navigation, inline patch viewing, guarded restore actions, commits, and commit identity selection.
• Adds expanded Git tools for status, branches, commits, staging/unstaging, restore, log, blame, worktrees, checkpoints, and account detection.
• Adds the top-bar branch switcher with current-branch priority, scrollable branch menu, pinned create-branch action, and inline branch creation.
• Adds Git settings for refresh behavior, restore confirmation, checkpoints, commit identity profiles, account switching, and detected Git/GitHub/GitLab identities.
• Adds a renderer ai.complete bridge for one-shot LLM completions and uses it to generate commit message suggestions from the full diff.
• Adds Stage all in the Git panel.
• Updates README, user/developer docs, and the web homepage/mockup for the new Git workflows.

Testing

• pnpm typecheck
• pnpm build
• pnpm test
• pnpm --filter @meith/desktop typecheck
• pnpm --filter @meith/web typecheck
• pnpm --filter @meith/desktop exec vitest run src/main/__tests__/agent.test.ts src/main/__tests__/gitTools.test.ts
• git diff --check