Add training tracking: exercises with instructions, one-tap session log, consistency overview

Exercises (name + how-to note) are a new synced collection with the same
LWW/tombstone contract as events, served by POST /api/exercises/sync.
Training sessions are ordinary events (type "training") referencing an
exercise by id, so they ride the existing event sync unchanged.

The Training panel lists each exercise with last-trained / this-week /
streak stats, expandable instructions, and a one-tap Log button with the
usual undo/add-note snackbar. An exercise-by-day heatmap shows the last
14 days of consistency, and history and the daily overview count
training sessions like any other event.

README.md

@@ -1,7 +1,7 @@
 # puppy-tracker
 
-A tiny offline-first PWA for tracking your puppy's sleep, meals, pees, poos, and
-weight.
+A tiny offline-first PWA for tracking your puppy's sleep, meals, pees, poos,
+weight, and training.
 The browser is the primary client; a small Go server provides a shared
 source-of-truth and sync between devices.
 
@@ -21,6 +21,11 @@ source-of-truth and sync between devices.
   `config.json` sitting alongside it, renaming them to `*.imported`.
 - Service worker bypasses cache for `/api/*` so writes always hit the server
   when online; static assets are still cached for offline use.
+- Training exercises (name + how-to instructions) are their own synced
+  collection with the same contract as events (UUIDs, last-write-wins,
+  tombstones) via `POST /api/exercises/sync`. Training sessions are ordinary
+  events (`type: "training"`) referencing an exercise by id, so they ride the
+  event sync unchanged.
 - The puppy's name and birthday are a per-account profile stored on the host
   (`GET`/`PUT /api/config`), so a new device picks them up automatically instead
   of being configured per-client. The client caches the last-seen values in