Add explicit PRD delivery order and fix epic sort ordering

[suprie]

Summary

• Adds an optional order: PRD frontmatter field (falls back to the epic README's ## PRD Index table position, then filename) so a PRD's shown delivery order no longer depends on alphabetical filename luck.
• New drag-to-reorder dialog in the per-project Epics tab (PrdReorderDialog.tsx), backed by two explicit backend commands: migrate_prd_order (backfill missing values, idempotent) and set_prd_order (full permutation-validated rewrite). Deliberately kept off the read path — an earlier version auto-backfilled on every read_prds call and corrupted this repo's own docs/epics/**/*.md the moment cargo test's dogfood test parsed real epics; reverted and redesigned as explicit, user-triggered writes only.
• Fixes two real epic-ordering bugs found along the way: EpicsView.tsx's cross-project milestone grouping claimed to already be sorted (only true per-project — merging projects preserved first-seen insertion order), and EpicsPanel.tsx's per-project epic list had no sort at all. Both now share one sortEpics helper (lib/utils.ts): status-primary (in_progress → proposed → abandoned → completed), milestone ascending (numeric, not lexicographic) secondary.
• The reorder dialog uses pointer events (pointerdown/pointermove/pointerup + pointer capture) instead of HTML5 draggable/dragover/drop — Tauri's native window-level drag-drop handling (already relied on by Chat.tsx attachments and ImportFlow.tsx repo import) intercepts real OS drag gestures before the webview's JS sees them, so the native-DnD version dragged visually but never persisted an order.

Why

User-reported: PRDs were rendering in the wrong delivery order (alphabetical filename, ignoring the epic's actual planned sequence), and epics themselves weren't consistently sorted by status/milestone across the two views that list them.

Test plan

• cargo test — 596 passed (61 in epic.rs, including new set_prd_order/migrate_prd_order/read-only-guard tests), 0 failed
• cargo clippy --lib -- -D warnings clean
• npx tsc --noEmit clean
• Isolated Node runs of the extracted sort logic against representative/screenshot-derived data, confirming both ordering fixes
• Live click-through in the running Tauri app — not verified this session (dev-server port held by another session throughout; the drag-to-reorder fix specifically needs the real webview since the bug it fixes only reproduces inside Tauri's native window, not a plain browser preview)