fix(dashboard): extract themed-dialog styles into an importable modal.css

The themed dialog component (modal.js: themedConfirm / themedPrompt /
themedToast) is raised from common.js's data-async / data-confirm
handler, which every page loads — but its .tc-* styles lived only in
dashboard.css (the main tabbed dashboard's stylesheet). So on any
standalone page (core/C0R3, settings, flow, logs) a dialog rendered
completely unstyled: the build-queue rebuild-cancel confirm came up as
raw text.

Extract the .tc-* rules into a dedicated modal.css component (paired
with modal.js) and @import it from common.css, so the styles load
wherever a dialog can fire — not just the main dashboard. esbuild
inlines the @import into the common.css bundle, so there's no extra
request and the main dashboard is visually unchanged (it loads
common.css too). The stale 'themedConfirm() in common.js' comment is
gone with the move. Fixes #1910.

frontend/packages/dashboard/src/common.css

@@ -4,6 +4,7 @@
 @import "@hive/shared/terminal.css";
 @import "@hive/shared/tabs.css";
 @import "@hive/shared/chrome.css";
+@import "./modal.css";
 
 /* ─── global typography ─────────────────────────────────────────────
    Element-level rules shared across all three pages (index, flow,