fix(repo-appearance): make canvas notification override repo tint
Cards on a repo with a pinned color rendered the unread-notification
state as orange blended *into* the repo color (e.g. blue + 0.30 orange
under .bar + 0.10 blue on top → muddy brownish-grey), so the
attention signal was harder to see than it was before this feature
shipped.
Restructure the title-bar background into mutually-exclusive top
layers: notification orange XOR repo identity tint, never both. The
notification jumps to a 0.55 alpha (vs the prior under-bar 0.30) so
the unread state pops cleanly without the repo color competing.
Selected-but-unfocused accent stays under .bar — its semantic is
"selection state", not "needs attention", and the existing subtle
treatment is correct.
For repos without a pinned color the behavior change is just "orange
notification is now more prominent than before"; for repos with a
color the muddy-blend is gone entirely.