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/commands.ts

@@ -78,7 +78,7 @@ export const COMMAND_DESCRIPTIONS: Record<string, { category: string; descriptio
   'dialog-accept': { category: 'Interaction', description: 'Auto-accept next alert/confirm/prompt. Optional text is sent as the prompt response', usage: 'dialog-accept [text]' },
   'dialog-dismiss': { category: 'Interaction', description: 'Auto-dismiss next dialog' },
   // Visual
-  'screenshot': { category: 'Visual', description: 'Save screenshot', usage: 'screenshot [path]' },
+  'screenshot': { category: 'Visual', description: 'Save screenshot (supports element crop via CSS/@ref, --clip region, --viewport)', usage: 'screenshot [--viewport] [--clip x,y,w,h] [selector|@ref] [path]' },
   'pdf':     { category: 'Visual', description: 'Save as PDF', usage: 'pdf [path]' },
   'responsive': { category: 'Visual', description: 'Screenshots at mobile (375x812), tablet (768x1024), desktop (1280x720). Saves as {prefix}-mobile.png etc.', usage: 'responsive [prefix]' },
   'diff':    { category: 'Visual', description: 'Text diff between pages', usage: 'diff <url1> <url2>' },