feat(dashboard): split monolithic CSS into common + per-page bundles

dashboard.css was 2500+ lines covering every page. Split into four
separate esbuild entry points:

- common.css  — @hive/shared imports + shared typography, badges,
                buttons, inbox/compose, side panel, diff panel,
                path linkification, logs-header chrome (shared by
                flow + logs)
- dashboard.css — dashboard-only styles (tabs, container rows,
                  approvals, questions, permissions, schedules, etc.)
- flow.css    — flow page chrome (frosted header, composer, inbox pill)
- logs.css    — log viewer sub-tabs (journal-*, build-logs-*)

Each HTML file now loads common.css + its own page-specific bundle.
build.mjs updated to produce all four CSS outputs. All builds pass.

Closes #1056

frontend/packages/dashboard/build.mjs

@@ -10,8 +10,12 @@
 //   dist/static/logs.js           /logs.html entry — build/agent/system
 //                                  log viewer sub-tabs
 //   dist/static/{tabs,flow,logs}.js.map source map siblings
-//   dist/static/dashboard.css     served at /static/dashboard.css
-//                                  (@import resolved from @hive/shared)
+//   dist/static/common.css        loaded by every page (@hive/shared
+//                                  imports + shared typography/badges/
+//                                  buttons/inbox/side-panel)
+//   dist/static/dashboard.css     /index.html only (dashboard-specific)
+//   dist/static/flow.css          /flow.html only (flow chrome + composer)
+//   dist/static/logs.css          /logs.html only (log viewer sub-tabs)
 //
 // Both JS entries inline `./common.js` (DOM helpers, Panel singleton,
 // NOTIF, path linkification) — esbuild dedupes the shared module
@@ -71,15 +75,19 @@ await build({
   logLevel: 'info',
 });
 
-// Bundle the CSS — esbuild resolves @import including the package
-// re-exports from @hive/shared.
-await build({
-  entryPoints: [src('dashboard.css')],
-  outfile: staticDir('dashboard.css'),
-  bundle: true,
-  loader: { '.css': 'css' },
-  logLevel: 'info',
-});
+// Bundle CSS — one entry per page. esbuild resolves @import including
+// the package re-exports from @hive/shared. Each page loads common.css
+// (shared typography, badges, buttons, inbox, side panel) plus its own
+// page-specific bundle.
+for (const entry of ['common.css', 'dashboard.css', 'flow.css', 'logs.css']) {
+  await build({
+    entryPoints: [src(entry)],
+    outfile: staticDir(entry),
+    bundle: true,
+    loader: { '.css': 'css' },
+    logLevel: 'info',
+  });
+}
 
 for (const html of ['index.html', 'flow.html', 'logs.html']) {
   copyFileSync(src(html), dist(html));