refactor(ui): read colours through context.colors, folder by folder (#1465) #1731

Part of #1465 (step 3b). Needs QueryaColors / context.colors (#1730).

Problem

Theme.of(context).colorScheme (shadcn Theme) is read about 710 times in lib/. Per folder (count of colorScheme / Theme.of( / shadcn.Theme on dev, 2026-10-18):

• features/connections (203)
• features/redis (96)
• features/mongodb (79)
• features/settings (77)
• features/workspace (72)
• core (72; core/theme last, with the shell)
• features/postgresql (39), features/mysql (26), features/sqlite (13)
• features/extensions (37)
• shared (17), features/main_screen (15), features/recovery (11), features/onboarding (9), features/erd (6), features/command_palette (6), features/updater (4), features/results (2), features/help (2), app (3)

Change

• Mechanical: final cs = Theme.of(context).colorScheme; → final cs = context.colors;; shadcn.Theme.of(context).colorScheme likewise; the field names stay.
• One PR per line of the list (the small folders together), each adding the folder to a guard test that forbids colorScheme / Theme.of( from shadcn there (the SQL pane chrome guard from epic(ui): SQL workspace in one visual system and with fewer control rows #1558 is the pattern).

Acceptance

• Every box ticked; the guard covers all of lib/ except core/theme and the shell, which go with epic(ui): remove shadcn_flutter from the app #1465 step 5.
• No visual change (goldens and widget tests pass unchanged).