feat: workspace git detection with branch/status badge

When the workspace root is a git repo, a badge in the panel header
shows the current branch name, dirty file count, and ahead/behind
status. Updates on every root directory load.

Backend:
- git_info_for_workspace() in api/workspace.py runs lightweight git
  commands (rev-parse, status --porcelain, rev-list) with 3s timeout
- New GET /api/git-info endpoint returns branch, dirty count, modified,
  untracked, ahead, behind

Frontend:
- _refreshGitBadge() in workspace.js fetches git info on root load
- Git badge element in panel header shows branch + status
- Badge turns gold when workspace has uncommitted changes

Inspired by PR #75 (@MartinNielsenDev).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/style.css

@@ -204,6 +204,8 @@
   .upload-bar{height:100%;background:linear-gradient(90deg,var(--blue),#a0d0ff);width:0%;transition:width .3s ease;}
   .rightpanel{width:300px;background:var(--sidebar);border-left:1px solid rgba(255,255,255,.06);display:flex;flex-direction:column;overflow:hidden;flex-shrink:0;}
   .panel-header{padding:12px 16px;border-bottom:1px solid var(--border);font-size:11px;font-weight:600;color:var(--muted);text-transform:uppercase;letter-spacing:.1em;display:flex;align-items:center;justify-content:space-between;}
+  .git-badge{font-size:9px;font-weight:600;color:var(--muted);background:rgba(255,255,255,.06);padding:2px 7px;border-radius:4px;letter-spacing:.02em;margin-left:auto;margin-right:4px;white-space:nowrap;font-family:'SF Mono',ui-monospace,monospace;}
+  .git-badge.dirty{color:var(--gold);background:rgba(201,168,76,.1);}
   .panel-actions{display:flex;gap:4px;}
   .panel-icon-btn{width:24px;height:24px;background:none;border:none;color:var(--muted);cursor:pointer;border-radius:5px;font-size:13px;display:flex;align-items:center;justify-content:center;transition:all .15s;}
   .panel-icon-btn:hover{background:rgba(255,255,255,.08);color:var(--text);}