feat(heph-tui,heph-pwa): humanized recurrence + indented/counted/scrolling project sidebar

Bundles the cosmetic/UI-polish backlog for the agenda surfaces. All read-side;
no schema or sync change (see hub-spoke-data-evolution).

- humanize_rrule (hephd::datespec): inverse of parse_recurrence — renders an
  RRULE as 'every other week', 'weekdays', 'yearly on Apr 15', etc.; falls back
  to the raw rule for unmodeled parts (COUNT/UNTIL/ordinal BYDAY). Mirrored in
  the PWA's datespec.js. Shown in the TUI recurs detail line and PWA task/qa
  previews instead of the raw FREQ= string.
- project.overview RPC + Store::project_overview: each project's parent (via the
  existing 'parent' links) and direct outstanding-task count, a read-only query.
- TUI sidebar: subprojects indented by depth, per-project counts, wider pane,
  and ListState + scrollbar so it scrolls instead of clipping on overflow.

Tests: humanize parity (Rust + JS), round-trip through parse_recurrence,
raw-passthrough; project_overview count/parent; sidebar tree ordering + cycle
safety.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/changelog.d/tui-polish-project-tree.doc.md

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+New explanation card [[hub-spoke-data-evolution]] covering why heph's op-based sync lets most new features ship without a coordinated migration, and the narrow case (a new required SQLite column) that does need a hub-first rollout.

docs/changelog.d/tui-polish-project-tree.feature.md

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+Recurring tasks now show their schedule in plain language (`every other week`, `weekdays`, `yearly on Apr 15`) instead of a raw RRULE — in both the TUI detail pane and the mobile PWA. The TUI's project sidebar gained subproject indentation, per-project outstanding-task counts, a wider pane, and scrolling when the list overflows.