Add missing test coverage for filters, Update handlers, time utilities, and cache concurrency

[joshuacrass]

Problem:
Current test coverage is limited to the data layer: cache, config, dashboard row building, branch name derivation, check summarization, truncation, JSON parsing, and markdown rendering. Zero BubbleTea Update()/View() logic is tested. Several utility functions and all filter resolution functions are untested.

Solution:
Add the following specific tests:

Filter resolution:

• TestResolveIssueFilters_Reset — actionChoice="reset" returns defaults
• TestResolveIssueFilters_CustomAssignee — choice="custom" uses AssigneeCustom value
• TestResolvePRFilters_DraftTrue / TestResolvePRFilters_DraftFalse — draft filter correctly mapped

Utilities:

• TestIsMeAssigned — empty login returns false; login present in list returns true
• TestTimeAgo — table-driven: past minute, hour, day, week, month, year; future timestamp; empty string; invalid string
• TestDeriveBranchName_Unicode — titles with accented characters, emoji, CJK

BubbleTea Update handlers:

• TestAppModel_Update_SwitchTab — dispatching switchTabMsg changes activeTab correctly
• TestIssuesModel_Update_KeyRefresh — r key sets loading=true and returns a fetch command
• TestIssuesModel_Update_FetchError — error msg sets m.err; subsequent refresh clears m.err

Concurrency:

• TestSaveCacheConcurrent — two goroutines call saveCache simultaneously; file is valid JSON afterward; no panic; go test -race passes

Acceptance Criteria:

• All tests listed above are implemented and pass
• go test -race ./... passes
• Test coverage of internal/github filter logic increases measurably