Feat/dynamic Save tabs button: "Save 1 tabs" button will now be displayed as "Save 1 tab"

[Praneeth003]

Fixes the Tabs “Save” button label to use correct singular/plural and extract the label builder for better readability.

Changes Show “Save 1 tab” when exactly one tab is selected; “Save N tabs” when N > 1. Implemented a small helper function buildSaveTabsLabel(t, count) in src/routes/extension/tabs/action.js to format the label. Kept all text localized via existing i18n keys.