fix(ui): mark sidebar focus with an accent colour, not bold (#25)

The focused sidebar panel border (ADR-0046 DC3) was bright `fg` plus
`Modifier::BOLD`. Bold box-drawing glyphs render as broken/gapped
line-art in the asciinema cast player and are fragile in some terminals.

`panel_border_style` now marks focus with a non-bold accent colour
(`theme.mode_simple`, blue); the unfocused border stays muted. Bold is
untouched on text spans (titles, key hints) — the constraint is
specifically that box-drawing borders carry no bold attribute.

Pure style change: the Tier-2 snapshots are text-only so none needed
re-accepting; the Tier-1 assertion was updated and a render-level test
now checks the rendered border cells carry the accent and no bold.

ADR-0046 Amendment 1.

docs/adr/0046-sidebar-navigation-and-responsive-input-hint.md

@@ -554,3 +554,27 @@ All tiers green, zero skips; clippy clean (nursery).
   and is accepted: 90 is the screencast width, real terminals sit well
   to one side of it, and `Ctrl-O` peek covers the in-between case. The
   `90` threshold is a tunable constant.
+
+## Amendment 1 — focus accent is a colour, not bold (2026-06-12)
+
+Issue #25. DC3's "accent border" on the focused sidebar panel was
+first implemented as bright `theme.fg` **plus `Modifier::BOLD`** on
+the box-drawing border. Bold box-drawing glyphs render as broken /
+gapped line-art in the asciinema player used for the website casts
+(vertical strokes don't connect to the corner glyphs) and are
+fragile in some terminals.
+
+**`panel_border_style` now marks focus with a non-bold accent
+colour — `theme.mode_simple` (blue) — and never `Modifier::BOLD` on
+a border.** The unfocused border stays muted `theme.border`. This
+makes the ADR's "accent border (lazygit convention)" wording
+literal — it is now a true accent hue rather than bold bright-fg —
+and is what renders cleanly in casts. Bold remains fine on *text*
+spans (titles, key hints); the constraint is specifically that
+box-drawing borders carry no bold attribute.
+
+Note: this is a pure style change. The Tier-2 snapshots are
+text-only (`render_to_string` captures cell symbols, not styles),
+so none needed re-accepting; the Tier-1 `panel_border_style`
+assertion was updated and a render-level test now checks the actual
+border cells carry the accent colour and no bold.