feat: last-session recall when starting an exercise

When you start an exercise, the Mini App now fetches the most
recent time you logged it and shows a hint line in the sets card
("Last time: 8×60, 6×60, 5×60 · 3 days ago"), plus pre-fills the
weight input with the last set's weight.

- db.get_last_exercise(user_id, name): most recent non-deleted
  entry, case-insensitive name match, sets_detail parsed.
- GET /api/exercises/last?name=<name>.
- webapp: loadLastSession() on startExercise + draft restore;
  hint cleared on editExercise (the set rows are the reference
  there). Pre-fill only when the weight field is empty and no
  sets logged yet, so it never clobbers user input.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

webapp/index.html

@@ -37,6 +37,7 @@
           </div>
           <button id="btn-delete-exercise" class="btn-link btn-danger hidden">Remove exercise</button>
         </div>
+        <div id="last-session-hint" class="last-session-hint hidden"></div>
         <div id="sets-list"></div>
         <div class="set-input-row">
           <input type="text" id="inp-reps" class="input input-small" placeholder="Reps" inputmode="numeric" pattern="[0-9]*" />