feat(shell): foldable sidebar groups, on-call pill, URL-held side panel - #2419

🟢 Confidence 10/10 · safe to merge. The four files changed since the last pass add pure helpers and nav rendering, each pinned by scene tests with an explicit now, so nothing depends on wall-clock time.
_{no findings · tests covered · risk medium}

The final pass on the shell rework: oncall.ts decides the pill's state, copy and re-read schedule, and view-nav.ts folds labelled groups while keeping the current row. Both are self-contained and tested; safe to merge.

• onCallStatusOf names the pill from the soonest-ending live shift, hides a stale Unknown
• scheduleAfterAnswer re-reads at the handoff or hourly, backing off a passed handoff
• navRow folds to the current sub-item under its section row when its group is folded
• navGroupFold keeps the collapsed rail unfolded, with plain labels as no tab stops

_{7360717 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}