Move sort control out of Issues/PR dropdowns into a filter popover

[gregpriday]

Summary

The recently added "Sort by" section in the Issues and Pull Requests dropdowns takes up persistent vertical space in the dropdown header. Sort and filter controls should live behind a filter icon button — matching the compact, consistent pattern used by the worktree sidebar.

Problem Statement

PR #3330 added a "Sort by" radio group that is always visible inside the dropdown header for both the Issues and PR panels. This creates two problems:

1. Wasted space — the sort section is permanently visible even when the user has no interest in changing it, shrinking the list area.
2. Inconsistent pattern — the worktree sidebar uses a search input + filter icon button that opens a popover for all sort/filter options. The GitHub dropdowns should follow the same pattern rather than inlining controls.

The worktree filter button (WorktreeFilterPopover) is a well-established pattern in the app: a compact icon that shows a badge when filters are active and opens a popover with sort and filter options. The GitHub resource panels should adopt the same affordance.

Evidence:

• src/components/GitHub/GitHubResourceList.tsx#L526-L568 — inline "Sort by" section always visible in the dropdown header
• src/components/Worktree/WorktreeFilterPopover.tsx — the compact filter icon + popover pattern to follow
• src/store/githubFilterStore.ts — sort order state already exists here

Desired Behavior

• The inline "Sort by" radio section is removed from the GitHubResourceList dropdown header.
• A small filter icon button appears next to the search input (mirroring the worktree sidebar layout).
• Clicking the filter icon opens a popover containing the sort order options (and any other per-list filters that may be added in future).
• When a non-default sort order is active, the filter icon shows an accent badge — consistent with how WorktreeFilterPopover signals active filters.
• The dropdown header is more compact as a result: just search + filter icon, with state tabs (Open/Merged/Closed) below.
• Sort order state persists exactly as it does today (via githubFilterStore).

Acceptance Criteria

• The "Sort by" radio section is no longer visible inline in the Issues or PR dropdown headers
• A filter icon button appears next to the search input in both dropdowns
• The filter popover contains sort order options (Newest / Recently updated) for the respective resource type
• The filter icon displays a visual indicator (badge or accent color) when a non-default sort order is active
• Sort order changes take effect immediately and re-fetch the list, same as today
• The dropdown header is visually more compact without the sort section

Edge Cases & Risks

• The state filter tabs (Open / Merged / Closed) should remain where they are — only the sort section moves.
• The filter popover should be dismissed by clicking outside or pressing Escape without losing the current sort selection.
• Non-default sort order badge should reset correctly when the user switches back to the default.