fix: three UI glitches -- image error, CLI badge overlap, dropdown z-index

1. Image preview onerror fires on clearPreview (#68)
   clearPreview() set previewImg.src='' which triggered the stale onerror
   handler, showing 'Could not load image' on every refresh/message.
   Fix: null out onerror before clearing src.

2. CLI session badge covers delete button (#69)
   The ::after 'cli' label occupied the same space as the hover-revealed
   .session-actions overlay, making delete unreachable.
   Fix: add padding-right to .cli-session, use margin-left:auto to push
   badge right, add pointer-events:none so clicks pass through.

3. Tool cards visible through profile dropdown
   The .messages container had no stacking context, so tool cards could
   render above the profile dropdown (z-index:200).
   Fix: add position:relative;z-index:0 to .messages to establish a
   stacking context that keeps all children below overlays.

Closes #68, closes #69

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

static/boot.js

@@ -169,7 +169,7 @@ $('importFileInput').onchange=async(e)=>{
 // btnRefreshFiles is now panel-icon-btn in header (see HTML)
 function clearPreview(){
   const pa=$('previewArea');if(pa)pa.classList.remove('visible');
-  const pi=$('previewImg');if(pi)pi.src='';
+  const pi=$('previewImg');if(pi){pi.onerror=null;pi.src='';}
   const pm=$('previewMd');if(pm)pm.innerHTML='';
   const pc=$('previewCode');if(pc)pc.textContent='';
   const pp=$('previewPathText');if(pp)pp.textContent='';