fix: code-in-table CSS sizing + markdown image rendering (#486, #487)

- static/style.css: add td code / th code rules (font-size 0.85em,
  padding 1px 4px, vertical-align baseline) for both .msg-body and
  .preview-md to fix cramped inline code in table cells (#486)

- static/ui.js inlineMd(): add image pass (![alt](url) → <img
  class=msg-media-img>) running while _code_stash is active (protects
  image syntax inside backticks), add _img_stash (\x00G) to shield
  rendered <img> src= from autolink, add img to SAFE_INLINE (#487)

- static/ui.js renderMd() outer: add image pass before outer link pass
  for images in plain paragraphs, add img to SAFE_TAGS allowlist (#487)

- tests/test_issue486_487.py: 45 new tests covering CSS source checks,
  JS source structure, rendering behaviour, and combination edge cases
  (code + image + link in same table cell, image inside code span, etc.)

Closes #486, closes #487

static/style.css

@@ -430,6 +430,8 @@
   .msg-body th{background:rgba(255,255,255,.07);padding:6px 10px;text-align:left;font-weight:600;border:1px solid var(--border2);}
   .msg-body td{padding:5px 10px;border:1px solid rgba(255,255,255,.06);}
   .msg-body tr:nth-child(even){background:rgba(255,255,255,.03);}
+  /* #486: inline code inside table cells needs scaled sizing to avoid overflow/clipping */
+  .msg-body td code,.msg-body th code{font-size:0.85em;padding:1px 4px;vertical-align:baseline;}
   /* KaTeX math rendering */
   .katex-block{display:block;text-align:center;margin:12px 0;overflow-x:auto;}
   .katex-inline{display:inline;}
@@ -589,6 +591,8 @@
   .preview-md th{background:rgba(255,255,255,.07);padding:6px 10px;text-align:left;font-weight:600;border:1px solid var(--border2);}
   .preview-md td{padding:5px 10px;border:1px solid rgba(255,255,255,.06);}
   .preview-md tr:nth-child(even){background:rgba(255,255,255,.03);}
+  /* #486: inline code inside table cells needs scaled sizing to avoid overflow/clipping */
+  .preview-md td code,.preview-md th code{font-size:0.85em;padding:1px 4px;vertical-align:baseline;}
   /* File type badge in preview path bar */
   .preview-badge{display:inline-block;font-size:10px;font-weight:600;padding:2px 6px;border-radius:4px;margin-left:8px;text-transform:uppercase;letter-spacing:.06em;}
   .preview-badge.img{background:rgba(124,185,255,.15);color:var(--blue);}