Fetch the viewer's push permission via GET /repos/{owner}/{repo} when
entering a repo and gate every write affordance behind it: edit and
image-upload buttons in StackedNote, new-fleeting-note and YouTube
buttons in FleetingNotes, and interactive checkboxes in TodoNotes
(rendered disabled when read-only). Anonymous viewers get the same
treatment because the permissions field is absent without auth, which
collapses to canPush = false.
Previously every write button was visible regardless of role, so
read-role collaborators and anonymous viewers could enter edit mode and
type before the save failed with 401/403.
updateFile/createFile now return { sha, conflict } so 409/422 from GitHub
can drive a UI flow instead of being swallowed as a generic save error.
Also adds fetchLatestSha(path) for cheap freshness checks against HEAD.
