Improve overflow handling for workspace tabs #11

Problem

When many workspace tabs are open, the top workspace bar becomes crowded. The tab strip technically supports horizontal scrolling, but the overflow affordance is hard to discover: the native scrollbar is hidden and the custom scrollbar is only a very thin thumb at the bottom of the tab strip.

As a result, users may not realize hidden tabs are scrollable and may feel that the only practical way to recover space or access earlier tabs is to close visible tabs.

Current behavior

• WorkspaceTabs renders open tabs inside a flex-1 horizontal scroll frame.
• The native horizontal scrollbar is hidden.
• Overflow is represented by a 4px custom scrollbar thumb.
• There is no explicit overflow menu, open-tabs list, scroll button, pinned/compact mode, or bulk tab management action.

Relevant implementation:

• packages/client/src/renderer/pages/workspace/components/workspace-tabs.vue
• packages/client/src/renderer/pages/workspace/composables/use-workspace-tabs.ts

Expected behavior

Users should have an obvious way to navigate, find, and manage tabs after the workspace tab strip overflows.

Possible UI approaches

1. Add an overflow tabs menu, shown only when tabs overflow. The menu can list all open tabs, highlight the active tab, and allow selecting or closing individual tabs.
2. Add left/right scroll buttons when overflow is present, scrolling by one tab or one viewport at a time.
3. Compress inactive tab widths before overflow, while keeping the active tab readable.
4. Add tab management actions such as Close other tabs, Close tabs to the right, and Close all except pinned.
5. Longer term: support pinned tabs for frequently used surfaces like Inbox, Review Queue, or key repositories.

Suggested direction

Start with an overflow tabs menu plus basic tab management actions. This keeps the existing scroll behavior, improves discoverability, and gives users a direct way to access hidden tabs without closing unrelated tabs.

Acceptance criteria

• When workspace tabs overflow, the UI exposes a visible navigation affordance.
• Users can access hidden tabs without closing visible tabs.
• Users can manage many tabs efficiently.
• The active tab remains easy to locate after opening or switching tabs.