docs(adr): ADR 0030 (Halkyon Train) and ADR 0031 (the apps' icons)

ADR 0030 records what Train does and where its data comes from: running,
cycling and rowing, in the order recording, intervals, routes, music; the
Polar Loop taken over through Polar's BLE SDK; music from Jellyfin;
devices on phones through flutter_reactive_ble; Health Connect both ways
through the health package; every row sealed; the WP 5000 an FTMS rower,
as scanned; no WaterRower USB driver for now; the Loop's firmware from
Polar directly or through the instance; flutter_map, the instance's map
source, BRouter as a sidecar, offline use and working with CoMaps; and
Train's shell after Todo's.

ADR 0031 gives the apps' icons one family: Lucide glyphs fading from
cobalt through a colour between into each app's own, Learn's graduation
cap, Todo's list, Train's bike, Meditate's circle-dot.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

docs/adr/0031-the-apps-icons.md

@@ -0,0 +1,57 @@
+# ADR 0031: The apps' icons, one family
+
+- **Status:** decided 28 Oct 2026, by the developer (D1–D4 below). Not built yet: Train's
+  icon comes with its shell (ADR 0030, D12), Learn's and Todo's follow, Meditate's with
+  Meditate.
+- **Builds on:** ADR 0030, D12 (Train's icon is Lucide's `bike`).
+
+## Context
+
+Halkyon's apps had no family. Learn's icon is the kingfisher, Halkyon's emblem (a
+picture, `halkyon_learn_flutter/assets/icon/halkyon_icon.png`); Todo's is Lucide's
+`list-checks`, its strokes drawn with Pillow in cobalt fading into sky
+(`apps/todo/tool/app_icons.py`). On 28 Oct, choosing Train's, the developer asked for
+"a fading of the colors, like in the icons of Proton's services", whose blog post
+(proton.me/blog/new-visual-universe) says: "Every icon starts with the Proton purple
+and then fades into the service color", so that the icons read as one family.
+
+Drawn and compared on 28 Oct (the previews in the session's scratchpad): fading from
+cobalt straight into one other colour looks, in the developer's words, "bland and odd",
+cobalt into sky "like everything is completely blue", and cobalt straight into an orange
+turns grey-brown halfway, as blue and orange are opposite colours. A gradient passing
+through a colour between them (violet or sky) stays bright.
+
+## Decisions (28 Oct 2026, the developer's)
+
+- **D1 Every app's icon is a Lucide glyph on white, fading from cobalt through a colour
+  between into the app's own**, top left to bottom right across Lucide's 24-unit box, at
+  Lucide's stroke width (2), the white tile's corners rounded by the platforms as today.
+  Cobalt is what the family shares, as Proton's purple; the end colours lie apart on the
+  colour wheel, so the icons are told apart small, the shapes doing the rest (Learn's and
+  Train's are related, as are Todo's and Meditate's; the developer kept them so).
+- **D2 The four**:
+
+  | App | Glyph | Gradient |
+  |---|---|---|
+  | Learn | `graduation-cap` | cobalt `#3C57E5` → violet `#8E4FD6` → strawberry `#EC3E7C` |
+  | Todo | `list-todo` | cobalt `#3C57E5` → pacific `#179FD9` → fern `#3CBB3A` |
+  | Train | `bike` | cobalt `#3C57E5` → violet `#8E4FD6` → strawberry `#EC3E7C` → carrot `#F78400` |
+  | Meditate | `circle-dot` | cobalt `#3C57E5` → pacific `#179FD9` → reef `#1DA583` |
+
+  The stops are evenly spaced. Cobalt is the icon's brighter cobalt Todo's icon uses, not
+  the palette's (`#273EB2`); violet is no palette colour (between cobalt and strawberry);
+  the others are halkyon_ui's palette's.
+- **D3 Learn's icon becomes the graduation cap; the kingfisher stays Halkyon's emblem**,
+  in the logo and the wordmark. Installed Learn apps change their icon once, when the new
+  one is built, as do F-Droid's listing and the snap's.
+- **D4 Meditate's glyph is `circle-dot`** (the developer's, over `flower-2`, recommended,
+  `wind`, `sprout`, `leaf` and `flower`): a point to rest the eyes on.
+
+## What follows (the agent's proposals, to be put to the developer when built)
+
+- One icon tool for every app at the repository's root, each app naming its glyph and
+  stops, instead of a script per app: Lucide's glyphs have arcs and curves (`bike`'s
+  wheels, `graduation-cap`), which Todo's tool, drawing straight strokes with Pillow,
+  can't draw. librsvg through PyGObject and pycairo (Ubuntu's `python3-gi`,
+  `gir1.2-rsvg-2.0`, `python3-cairo`) renders them exactly; the previews were drawn so.
+- Lucide's licence (ISC) beside each icon and in each app's licence list, as Todo's is.