fix(linux): restore GTK divider colors from active theme

[turinglambdaai]

Bug

The GTK stylesheet builds the .taskly-divider rule using a single quoted C++ string, so its CSS background value is literally ' + std::string(p.divider) + ', not the selected palette color. GTK cannot interpret this color and the divider may render with an unintended default/transparent style.

Fix

Interpolate p.divider into the generated CSS string with valid C++ concatenation. This preserves the intended theme token across light/dark appearance and fixes the 1px separator rendering.

Verify before merge

• Linux host build and packaged Product Acceptance pass
• Inspect sidebar/pane divider and task-input separator in light and dark themes
• Confirm the GTK CSS provider emits no parser warning for .taskly-divider

Independent of #33 (Linux multi-selection); no release or merge until integrated verification.