design(mytasks): remove stray divider lines from the group-by segment

[johnpc]

Design-polish pass — a mobile-zoom defect hunt on My Tasks surfaced stray vertical lines in the "Group by" segment. Between the tabs (Priority | Focus | Project | Label) sat thin divider rules that read as leftover artifacts in what's meant to be a clean pill-style toggle.

Root cause

Ionic's iOS ion-segment-button draws an inter-button divider via its shadow-DOM :host::before (a border-left), controlled by the documented --border-color custom property.

Fix (CSS-only, tokens only)

Set --border-color: transparent on .mytasks__group ion-segment-button to drop the dividers. The segment now reads as a clean toggle — active state intact (blue underline on desktop, white pill on mobile). No divider lines on either viewport.

Before → after

• Mobile (no dividers between tabs): https://files.jpc.io/d/2XTnY-groupby-no-dividers-mobile.png
• Desktop: https://files.jpc.io/d/8uj39-groupby-no-dividers-desktop.png

Full quality gate green (85 My Tasks tests pass); presentational-only. Verified against the sandbox (AWS_PROFILE=personal).