Switch to Inter, round the corners softly and make the nav switch tabs

Inter replaces General Sans (Inter Display for titles and big numbers).
Corners come back from before the squaring commit, scaled to a soft set
(16 cards, 14 buttons, 12 rows, 8 chips) and applied to the screens built
since. The nav is now a cluster of round frosted buttons that switches
Today, Routines, Goals, Groups and Progress in place inside Home: no
pushed pages, no back arrows, and Android back returns to Today.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KhQh614DMeYoQEH9SoMRUw

frontend/design.md

@@ -30,7 +30,11 @@ build their own layouts from the same pieces.
 The sections below describe the original record-style type scale, which still
 applies to labels and metrics.
 
-## Type — General Sans
+## Type — Inter
+
+`TextType.Light` is Inter Display Regular (the display cut, for titles and
+big numbers); Regular, Medium and Bold are Inter Regular, Medium and
+SemiBold.
 
 | Role | Family | Size | Use |
 |---|---|---|---|
@@ -45,7 +49,8 @@ directly above a large dark value. Used everywhere numbers appear.
 
 ## Geometry
 
-- No rounded corners anywhere: sheets, cards, chips, buttons, fields, tiles and portraits are all square (`BorderRadius.zero`). Only small status dots, glows, the sun, progress rings and the breathing circle stay round.
+- Softly rounded, never pill-soft: large cards and image cards `16`, buttons `14`, rows, tiles and fields `12`, small controls `10`, chips and badges `8`, bars `4`. Sheets round their top at `20`.
+- Portraits, the nav buttons, status dots, rings and the breathing circle are full circles.
 - Screen padding `20` horizontal. Card padding `16`. Section gap `28`.
 - Hairlines at `colorDivider` (black 7%), borders at `colorBorder` (black 8%).
 - One soft shadow only: `0 4 18 rgba(0,0,0,0.03)`. No coloured glows.
@@ -66,6 +71,14 @@ Near-black ink (`colorPrimaryDark #141414`) on warm off-white
 The tier colour is used consistently — header, chip, meter, queue accent — so
 the standing is legible without reading the word.
 
+## Navigation
+
+The nav is a floating cluster of round frosted buttons, icons only; the
+open tab is a raised white disc. It switches tabs inside Home (Today,
+Routines, Goals, Groups, Progress) in place: nothing is pushed, a tab has
+no back arrow (`tab: true`), and Android back returns to Today. Tabs are
+built on first open and then kept.
+
 ## Structure
 
 Every screen is: black `headerBar` (back, title, action) → white sheet with a