fix(sidebar): stop the nav column painting over the profile block

The account-menu swap only covered the popup case. A fresh owner
screenshot reproduces the overlap with no popup open: on a short viewport
the "Assistant History" header renders behind the bottom profile row.

Root cause is the flex chain, not the dropdown. The middle region is
flex-1 min-h-0, so it shrinks to nothing when the fixed nav sections
above it (this pilot user is a firm admin, so Firm *and* Firm admin are
both present) eat the height. Its children then keep their intrinsic
height - only the row list is overflow-y-auto, the Recent Matters block
and the History header are not - and with no clipping ancestor they paint
straight through onto the mt-auto profile block.

Wrap the nav sections and the middle region in one `flex-1 min-h-0
overflow-y-auto` column. The profile block is now a SIBLING of a scroll
container, so nothing above it can paint over it at any height - an
invariant of the structure rather than of the arithmetic. Add a min-h-56
floor on the middle region so History degrades to a shorter scrollable
list instead of a half-rendered header, and shrink-0 on the profile.

Verified by headless-Chrome renders of a raw-CSS harness translating this
class chain, before vs after, with the full firm-admin nav:
  - 820px viewport (the reproducing config): before overlaps, after clear
  - 700px viewport: before spills outside the card entirely, after clear
  - tall viewport: before/after pixel-identical, no scrollbar, no
    regression to the pinned Recent Matters + inner history scroll

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>