feat(overview): add a Starred filter to the file list - #127

1. OfficeOverview.vue's new button and heading are untested (medium)

OfficeOverview.spec.ts is untouched by this commit. Nothing currently fails if the new "Starred" button were deleted, wired to the wrong filter value, or the heading text were wrong — findButtonByText is exact-match and no test asserts button count or heading text. The underlying filterFiles() logic is solidly tested; the gap is specifically in the component wiring around it. Suggest a test mounting the overview with a mix of starred/unstarred files, clicking (or setting) the Starred filter, and asserting both the filtered result and the "Starred {category}" heading text.

2. The "show all in Files" escape hatch ignores the active filter (low)

The overview's DAV search is capped at 500 results (SEARCH_RESULT_LIMIT), so "Starred" only searches the 500 newest office files — an older favourite outside that window won't show up here. That's an existing constraint shared with mine/shared, but for those filters openInFiles()'s fallback (linking to /apps/files/recent) at least lands somewhere plausible. For "Starred" specifically, a user hitting the cap has no path to see their older starred files — openInFiles() always links to /apps/files/recent, not a starred-filtered Files view, so the fallback doesn't actually answer the problem for this filter. Not asking to fix the 500-item cap here — just noting the escape hatch is a dead end specifically for this filter, worth a follow-up if it's not already covered by #99.