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/CLAUDE.md

@@ -111,7 +111,8 @@ status field. That's what makes honest history and excuse analysis possible.
 
 See `design.md`. In short: sage-to-cream ambient backdrop, frosted glass
 controls, painted `SceneArt` instead of photos, image-led cards, two-tone
-greetings, square night buttons with a lime spark, no rounded corners.
+greetings, night buttons with a lime spark, softly rounded corners (never
+pill-soft) and a floating round-button nav that switches tabs in place.
 
 - Screens are built from `Sheet` (`designs/Shell.dart`); Home, Routines and
   the routine player compose `AmbientBackground` + glass directly.
@@ -121,7 +122,7 @@ greetings, square night buttons with a lime spark, no rounded corners.
 - Standing owns a colour used consistently everywhere (chrome, chip, meter).
 - One primary button per screen; everything else outlined or text.
 
-Font: **General Sans** (Fontshare, FFL — see `fonts/LICENSE-GeneralSans.txt`),
+Font: **Inter** (SIL OFL — see `fonts/LICENSE-Inter.txt`; Light is Inter Display),
 wrapped as `TextType.{Bold,Medium,Regular,Light}`.
 
 ---