fix(chat): forward polish + scrollbar + image preview + media tabs

- #4: forward modal buttons now branded (bzc-* un-scoped from .bz-confirm); target
  rows get real checkboxes + border highlight; selected MESSAGES highlight in green
  with a filled tick (distinct from blue bubbles), checkbox on every message.
- #5: forwarded messages show an italic 'Forwarded from <origin>' label; new fwd_from
  column preserves the true origin across re-forwards.
- #6: image-preview arrows are smaller (32px) chevron icons, clear of the image
  (image max 82vw); icons.js?v=5.
- #10: image preview now opens ABOVE the media modal (z 9900) and Esc/close returns
  to the media view instead of closing everything.
- #11: Media/Links/Docs active tab underline is brand-blue, not green.
- #2: chat-list + conversation scrollbars 6px, stepper arrows hidden; pagination adds
  a cooldown + overflow-anchor:none so scrolling up no longer sticks/jumps.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/public/icons.js

@@ -40,6 +40,8 @@
     pencil:      '<path d="M21.17 6.83a2.83 2.83 0 0 0-4-4L3.84 16.17a2 2 0 0 0-.5.83l-1.32 4.35a.5.5 0 0 0 .62.62l4.35-1.32a2 2 0 0 0 .83-.5z"/><path d="m15 5 4 4"/>',
     chevronDown:    '<path d="m6 9 6 6 6-6"/>',
     chevronUp:      '<path d="m18 15-6-6-6 6"/>',
+    chevronLeft:    '<path d="m15 18-6-6 6-6"/>',
+    chevronRight:   '<path d="m9 18 6-6-6-6"/>',
     star:        '<path d="M11.525 2.295a.53.53 0 0 1 .95 0l2.31 4.679a2.123 2.123 0 0 0 1.595 1.16l5.166.756a.53.53 0 0 1 .294.904l-3.736 3.638a2.123 2.123 0 0 0-.611 1.878l.882 5.14a.53.53 0 0 1-.771.56l-4.618-2.428a2.122 2.122 0 0 0-1.973 0L6.396 21.01a.53.53 0 0 1-.77-.56l.881-5.139a2.122 2.122 0 0 0-.611-1.879L2.16 9.795a.53.53 0 0 1 .294-.906l5.165-.755a2.122 2.122 0 0 0 1.597-1.16z"/>',
     link:        '<path d="M10 13a5 5 0 0 0 7.54.54l3-3a5 5 0 0 0-7.07-7.07l-1.72 1.71"/><path d="M14 11a5 5 0 0 0-7.54-.54l-3 3a5 5 0 0 0 7.07 7.07l1.71-1.71"/>',
     layoutDashboard:'<rect width="7" height="9" x="3" y="3" rx="1"/><rect width="7" height="5" x="14" y="3" rx="1"/><rect width="7" height="9" x="14" y="12" rx="1"/><rect width="7" height="5" x="3" y="16" rx="1"/>',