Consolidate duplicated clock-time formatting helpers in the web app

[RobinEdquist]

The H:MM:SS clock-time formatter is copy-pasted in at least five places in apps/web, with slightly diverging details (one copy lacks the isFinite guard):

• components/player/player-bar.tsx — formatTime()
• components/player/chapter-drawer.tsx — formatDuration() (identical copy)
• app/(authenticated)/audiobooks/[id]/page.tsx — formatChapterTime() (no isFinite guard)
• components/users/session-log.tsx — formatPosition() + formatSessionDuration()

The shared lib/format-duration.ts only covers humanized durations ("5h 30m"), not clock-style timestamps.

The bookmarks feature (#64) introduces a shared apps/web/lib/format-timestamp.ts with formatTimestamp(seconds) (and parseTimestamp() for input). Once that lands:

Task: migrate the five local copies to the shared formatTimestamp() util and delete the local helpers. Behavior should stay identical (H:MM:SS above one hour, M:SS below, "0:00" fallback for non-finite input).

Acceptance criteria:

• No local clock-time formatters remain in the files listed above
• All call sites use lib/format-timestamp.ts
• Existing unit tests pass; format-timestamp tests cover the fallback cases the local copies handled