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/load_test.go

@@ -106,6 +106,12 @@ sev_critical: "#000009"
 sev_error: "#00000a"
 sev_warning: "#00000b"
 sev_info: "#00000c"
+identity_1: "#00000d"
+identity_2: "#00000e"
+identity_3: "#00000f"
+identity_4: "#000010"
+identity_5: "#000011"
+identity_6: "#000012"
 `)
 
 	got, err := loadFrom(dir, "full")
@@ -115,6 +121,9 @@ sev_info: "#00000c"
 	if got.Primary != "#000001" || got.SevInfo != "#00000c" {
 		t.Errorf("tokens not applied: %+v", got)
 	}
+	if got.Identity[0] != "#00000d" || got.Identity[5] != "#000012" {
+		t.Errorf("identity tokens not applied: %+v", got.Identity)
+	}
 }
 
 func TestLoadFrom_Errors(t *testing.T) {