Clear m.err when starting a list refresh

[joshuacrass]

Problem:
In model_issues.go and model_prs.go, when a fetch fails m.err is set and the error box is displayed. When the user presses r to refresh, m.loading = true and m.loaded = false are set, but m.err is not cleared. If the user switches tabs and comes back before the refresh completes, silentFetchCmd fires (because m.loaded == true) but the stale error box persists until the new data arrives.

Solution:
Add m.err = nil alongside m.loading = true in the refresh key handler in both model_issues.go and model_prs.go.

Acceptance Criteria:

• m.err is set to nil whenever a manual refresh (r) is triggered
• The error box disappears immediately when refresh is triggered, replaced by the loading spinner
• Fix applied to both IssuesModel and PRsModel