frontend: move shared/tabs.js+css into one-dir tabs/

Pure structural move, no API or behaviour change: tabs.js/tabs.css
move into shared/src/tabs/, matching the one-dir-per-component
layout the other shared components already use (hive-btn,
hive-dialog, hive-toast, hive-menu, side-panel).

Both files are consumed exclusively through @hive/shared's
package.json exports map (./tabs.js, ./tabs.css), never by a raw
relative path, so updating the two export targets is the only
change needed -- none of the 9 call sites (dashboard tabbar, logs,
core, builds, credentials, stats x2, agent stats) touch anything.
Verified the built dashboard/agent bundles still resolve both
files correctly.

frontend/packages/shared/package.json

@@ -8,8 +8,8 @@
   "exports": {
     ".": "./src/index.js",
     "./terminal.js": "./src/terminal.js",
-    "./tabs.js": "./src/tabs.js",
-    "./tabs.css": "./src/tabs.css",
+    "./tabs.js": "./src/tabs/tabs.js",
+    "./tabs.css": "./src/tabs/tabs.css",
     "./colors.css": "./src/colors.css",
     "./theme.css": "./src/theme.css",
     "./base.css": "./src/base.css",