feat(stats): add stats page and pile trend

Show the stat groups on /stats, the pile trend on the dashboard card,
one switch per group in the settings, and the shared groups on the
public profile.

README.md

@@ -19,12 +19,16 @@ To Be Played is split into three repositories:
 
 ## Features
 
-- **Dashboard**: most played games, games in progress, "Pile of Shame" with backlog hours and a random suggestion,
+- **Dashboard**: most played games, games in progress, "Pile of Shame" with backlog hours, its trend of the last 90
+  days ("this month: +N added, -N off the pile") and a random suggestion,
   Steam avatar and a banner from the most played game of the last two weeks, and an achievement card: unlocked
   total, perfect games, average completion and the 5 games closest to 100 %.
 - **What to play next** (`/play-next`): three backlog picks for a time budget (up to 5 / 15 / 40 h or any) and a mood
   (story, co-op, chill, action, scary from IGDB; speedrun from speedrun.com), high priority first, reroll without
   repeats, and "Start playing" in one click.
+- **Your stats** (`/stats`): totals and statuses; the pile in hours, in weeks at your pace of the last 90 days and in
+  years of full-time work, and its oldest unplayed game; most played and most loyal games; top genres and moods;
+  trips to the moon, the longest gaming day, games started but under one hour; finished games per year.
 - **Library**: status filters, "short games" filter (up to 10 h), search, sorting by playtime, recently seen, newest
   in library, rating or priority, paging. Each row shows the unlocked achievements.
 - **Add a game by name** for games outside Steam (PlayStation, Switch, Xbox, physical): search IGDB, pick a hit,
@@ -43,6 +47,7 @@ To Be Played is split into three repositories:
   the proved Steam ID, never a typed-in one. A Steam ID typed in before shows as "not verified" with a button to
   verify it; unlinking is possible.
 - **Public profile** at `/u/<display name>`, opt-in in the settings, with perfect games and average completion.
+  The user picks in the settings which stat groups of "Your stats" it shows (none by default).
 - **Appearance**: accent color, corner style, wide layout, page background and banner from your own game art.
   Saved in Auth, so it follows you to every device.
 - **Broken pictures**: a Steam cover or achievement icon that does not load shows a placeholder and is reported