fix: sync nav timer with timer mode and checklist mode

[hyldmo]

Summary

• Nav RestTimer now shows set elapsed when a set timer is active, matching TimerMode's display
• Session page passes startedAt to the store so session elapsed timer works in checklist mode (not just timer mode)
• init() preserves existing rest timers when entering timer mode for the first time

Test plan

• In timer mode: start a set, exit → nav shows same set elapsed time
• In timer mode: rest countdown running, exit → nav shows same countdown
• In checklist mode: confirm a set → rest countdown appears in nav
• In checklist mode: rest expires → nav shows session elapsed (not just dumbbell)
• Start rest in checklist mode, then open timer mode → rest countdown persists

🤖 Generated with Claude Code

[github-actions]

Preview deployment

URL	https://hyldmo-fix-timer-sync.macromaxxing.pages.dev
Branch	hyldmo/fix-timer-sync
Commit	296340a