On the mobile sidebar header the close (X) button rendered an 18px glyph — the
only 18px .panel-head-btn icon — sitting ~6px lower than the adjacent 14px
new-conversation (+) button, an inconsistent pair. Match the X glyph to 14px and
vertically center it on the ~41px panel-head row so both icons share one size and
baseline (top:calc(safe-top - 2px) keeps the existing safe-area offset so the X
still clears the notch, with no extra drawer spacing). 44x44 tap target kept for
mobile touch; mobile @media(max-width:640px) only, and the X is display:none on
desktop, so desktop is unaffected. + CHANGELOG + source-guard tests.
