fix(ui): constrain slash autocomplete width to composer — v0.50.98 (closes #633) (#750)

## Summary

Rebased-on-behalf of @franksong2702 (originally PR #728 — had CHANGELOG conflict after #747 merged).

Moves `#cmdDropdown` from outside `composer-box` to inside it, so the `position:absolute` anchor is scoped to the composer width rather than the full chat panel. CSS updated to use `bottom:calc(100% + 4px)` and `width:auto;max-width:100%` for clean upward positioning.

Closes #633

## Changes
- `static/index.html` — moved `cmd-dropdown` div inside `composer-box`
- `static/style.css` — updated `.cmd-dropdown` positioning (remove `margin-bottom`, use `bottom:calc(100% + 4px)`, add `width:auto;max-width:100%`)
- `tests/test_sprint50.py` — 2 new structural tests verifying DOM position and CSS rules

## Tests
1493 passed, 1 warning (2 new tests added)

**Original author:** @franksong2702

static/index.html

@@ -226,7 +226,6 @@
       </div>
     </div>
     <div class="composer-wrap" id="composerWrap">
-      <div class="cmd-dropdown" id="cmdDropdown"></div>
       <div class="composer-flyout">
       <div class="approval-card" id="approvalCard" role="alertdialog" aria-labelledby="approvalHeading" aria-describedby="approvalDesc">
         <div class="approval-inner">
@@ -275,6 +274,7 @@
       </div>
       </div>
       <div class="composer-box" id="composerBox">
+        <div class="cmd-dropdown" id="cmdDropdown"></div>
         <div class="drop-hint" id="dropHint">
           <svg width="28" height="28" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5"><path d="M21 15v4a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2v-4"/><polyline points="17 8 12 3 7 8"/><line x1="12" y1="3" x2="12" y2="15"/></svg>
           Drop files to upload to workspace