refactor(frontend): unify Tailwind classes — replace hardcoded colors with semantic design tokens

- Replace all bg-blue-600/hover:bg-blue-700 → bg-primary/hover:bg-primary-hover
- Replace all bg-red-600/hover:bg-red-700 → bg-destructive/hover:bg-destructive-hover
- Replace all focus:ring-blue-500 → focus-visible:ring-primary-ring
- Replace all text-blue-600 → text-primary on interactive elements
- Replace all bg-blue-50 → bg-primary-light on UI surfaces
- Replace all bg-red-50 → bg-destructive-light on error surfaces
- Replace all border-blue-200/300 → border-primary-ring
- Replace all border-red-200 → border-destructive-light
- Replace peer-checked:bg-blue-600 → peer-checked:bg-primary (toggles)
- Replace focus:outline-none focus:ring-2 focus:ring-offset-2 focus:ring-blue-500 patterns
- Keep status badge patterns (bg-*-100 text-*-700) and hover:bg-blue-50 intact

82 files changed, ~400 changes. Build passes.

frontend/src/components/git/GitMergeDialog.svelte

@@ -110,7 +110,7 @@
           onclick={onAbortMerge}
           disabled={mergeAbortInProgress}
           isLoading={mergeAbortInProgress}
-          class="border border-red-300 text-red-700 hover:bg-red-50"
+          class="border border-destructive-ring text-red-700 hover:bg-red-50"
         >
           {$t.git?.unfinished_merge?.abort_merge || 'Abort merge'}
         </Button>