refactor(connections): colours through context.colors, with a guard (#1731, connections)

[ZhuchkaTriplesix]

Part of #1731 (first folder: features/connections, ~200 reads). Epic #1465.

Mechanical, no visual change:

• final theme = Theme.of(context); + theme.colorScheme.x → final colors = context.colors; + colors.x (15 files);
• Theme.of(context).colorScheme → context.colors (ssh_tunnel_section, environment_section);
• the sidebar folder menu takes QueryaUiColors instead of shadcn's ThemeData;
• driver_icon read Material's colorScheme.primary, which querya_material_theme.dart maps from the same primary; it reads context.colors.primary now.

context.colors reads the shell theme while there is one (#1730), so every value is the one the code read before, mid theme animation included.

New guard colors_through_context_test.dart: in the folders it lists, no colorScheme, Theme.of( or ColorScheme. It starts with lib/features/connections; each next folder of #1731 adds itself in its PR.

dart analyze is clean; guards, connections, main-screen (goldens included) and e2e suites pass locally (311).