Show a colour-coded team picker instead of cycling with T

T used to silently cycle Model.activeTeamID through the caller's teams with
no visible list of choices. It now opens a full picker (modelled on the
existing user picker) listing every team plus "All teams", each with a
stable identity colour from a new six-colour theme palette. The same colour
now also shows as a bullet next to "team: <name>" in the header, so the
active team stays visible without opening the picker.

Adds an Identity palette to the theme package (six hues, skipping the ones
that already mean firing/critical), Styles.TeamColor to pick one by team id,
and identity_1..6 as theme-file tokens alongside the existing twelve so a
fully custom theme can still set every token.

internal/theme/theme.go

@@ -29,4 +29,8 @@ type Theme struct {
 	SevError    lipgloss.Color
 	SevWarning  lipgloss.Color
 	SevInfo     lipgloss.Color
+
+	// Identity tells things like teams apart from one another — not a status
+	// or a severity, so none of the six carries a meaning of its own.
+	Identity [6]lipgloss.Color
 }