SegmentCreate: prompt after save instead of immediate redirect; update plan

After saving, show "Saved! Add another from this activity?" with two
buttons: "Add another" (resets name/handles, keeps map loaded) and
"Done" (navigates to /segments/).

segments-plan.md

@@ -187,3 +187,46 @@ UI-first, so users can start populating the segment collection immediately. Dete
 11. `/segments/{id}/` page — segment detail + effort history table
 12. Activity detail — show matched segment efforts below stats
 13. Athlete page — segments section
+
+---
+
+## Phase 4 Detail
+
+### New API endpoints
+
+| Endpoint | Notes |
+|---|---|
+| `GET /api/segments/{id}` | Single segment metadata |
+| `GET /api/activities/{activity_id}/segment_efforts` | Scans user's effort files; returns `[{segment_id, segment_name, elapsed_s, started_at, ...}]` for hits on this activity. Auth required. |
+| `GET /api/users/{handle}/segment_summary` | Public. Returns `[{segment: {...}, best_elapsed_s, effort_count}]` — all segments where this user has efforts, best time + count each. |
+
+### SegmentDetail page (`/segments/{id}/`)
+
+- Map with segment polyline (reuse MapLibre, same style as `SegmentsView`)
+- Metadata row: name, sport, distance, created by
+- Effort history table sorted newest-first; columns: Date, Time, Avg speed, Avg HR, Avg power, NP, Δ vs PR
+  - PR row highlighted; Δ column shows `+N s` / `-N s` in red/green vs best effort
+- "Retrigger detection" button with loading state (blocks, shows count when done)
+- Auth-gated: effort table only rendered if logged in
+
+### Activity detail additions
+
+Below the laps table: if the user is logged in and this activity has any segment effort hits, show a compact block — segment name (linked to `/segments/{id}/`) + elapsed time + Δ vs PR.
+
+Fetch: `GET /api/activities/{activity_id}/segment_efforts` (gracefully hidden if 401 or empty).
+
+### Athlete page additions
+
+New "Segments" tab in `AthleteView.svelte`. Fetches `GET /api/users/{handle}/segment_summary` (public, no auth needed). Shows a table: Segment name (linked), Sport, Distance, Best time, # efforts. Empty state if no efforts yet.
+
+### Effort time display
+
+Use compact `m:ss` format (e.g. "5:23") for segment effort tables — add `formatElapsed(s: number): string` to `format.ts`.
+
+### Post-save UX in SegmentCreate (updated)
+
+After a successful save, instead of immediately redirecting to `/segments/`, show an inline prompt:
+> Segment saved! Add another from this activity?  [Yes, add another] [Done]
+
+- **"Yes, add another"** — clears `segName`, `segSport`; resets handles to full range; keeps the map and activity loaded
+- **"Done"** — navigates to `/segments/`