feat: screenshot element/region clipping (v0.3.7) (#56)

* feat: screenshot element/region clipping (--clip, --viewport, CSS/@ref)

Add element crop (CSS selector or @ref), region clip (--clip x,y,w,h),
and viewport-only (--viewport) modes to the screenshot command. Uses
Playwright's native locator.screenshot() and page.screenshot({ clip }).
Full page remains the default. Includes 10 new tests covering all modes
and error paths.

* chore: bump version and changelog (v0.3.7)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add screenshot modes to BROWSER.md command reference

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

browse/src/cli.ts

@@ -283,7 +283,8 @@ Inspection:     js <expr> | eval <file> | css <sel> <prop> | attrs <sel>
                 console [--clear|--errors] | network [--clear] | dialog [--clear]
                 cookies | storage [set <k> <v>] | perf
                 is <prop> <sel> (visible|hidden|enabled|disabled|checked|editable|focused)
-Visual:         screenshot [path] | pdf [path] | responsive [prefix]
+Visual:         screenshot [--viewport] [--clip x,y,w,h] [@ref|sel] [path]
+                pdf [path] | responsive [prefix]
 Snapshot:       snapshot [-i] [-c] [-d N] [-s sel] [-D] [-a] [-o path] [-C]
                 -D/--diff: diff against previous snapshot
                 -a/--annotate: annotated screenshot with ref labels