feat(ui): files-uploaded stat card with sparkline and smoother Activity load-in

[pmaxhogan]

Feature A: files-uploaded stat card with sparkline

A new Files uploaded tile sits immediately after the throughput tile in the
Activity header, in the same Grafana stat-panel style: a headline count with the
last 5 minutes of per-bucket file counts drawn behind it.

The chart itself was extracted rather than duplicated. SparklineStatTile.vue
now owns the geometry, the marks, the hover crosshair/readout and the empty
state; ThroughputStatTile.vue became a thin wrapper that turns bytes into a
rate, and FilesUploadedStatTile.vue is its sibling that keeps a count a count
(a bucket reads as "6 files", never "0.6 files/s"). Counts are pluralized and
grouped through Intl.NumberFormat, all strings via vue-i18n.

Both tiles read one window: the headline is the summary's new
throughputWindowFiles, the same window throughputWindowBytes already
covered, and both sparklines come from one query over one bucketisation.

Wire-shape change (breaking, in-tree only)

activity_throughput_series now returns { bytes: u64[], files: u64[] }
instead of a bare u64[]. Both arrays are dense, oldest-first and the same
length, so bucket i means the same slice of time in each. The frontend is the
only caller and is updated in this PR; the store treats a bare-array response
(a skewed backend) as "no data" rather than plotting indices.

Bug fixed along the way

activity_summary and the series both filtered event_type = 'upload_done',
which silently dropped every bundle_upload row - the V2-bundling rows that
pack N small files into one object. A source whose files get bundled therefore
under-reported "Uploaded today / this week" and read as idle on the throughput
tile. Both queries now match event_type IN ('upload_done', 'bundle_upload'),
and file counts use COALESCE(file_count, 1) so a plain upload counts as one
file and a bundle counts as all of its members. Covered by new tests.

Feature B: smoother Activity load-in

Switching to the Activity tab painted empty tiles, an empty filter bar and
"Showing 0 of 0", then swapped each one out as its query landed. The view now
holds a skeleton of its own shape (tiles, filter bar, table) until the first
load settles - in a finally, so a failed load falls through to its error state
rather than pulsing forever - then fades the real content in with a one-shot CSS
animation. prefers-reduced-motion disables both the fade and the pulse.

A plain keyframe rather than <Transition>: the content enters a fragment that
was never in the DOM, so there is nothing to transition from, and a keyframe
cannot get stuck mid-flight.

Also

loadEventTypeOptions now coerces a non-array response to []. An
unregistered/skewed command resolves undefined instead of rejecting, which
crashed the view's eventTypeOptions.length read on the next render.

Tests

• cargo test -p driven-core state:: - 83 passed, 0 failed
• vitest run - 365 passed, 0 failed (38 files)
• cargo clippy --workspace --all-targets - no warnings
• cargo fmt --all, eslint . (0 errors), vue-tsc --noEmit clean
• .sqlx/ offline cache regenerated via just sqlx-prepare

New: sqlite tests for per-bucket file counts, bundle rows counting all members,
and the summary's window file count; mount tests for SparklineStatTile and
FilesUploadedStatTile; an Activity.vue test covering the skeleton, the
failed-load path and the new tile's position and headline.

🤖 Generated with Claude Code

[github-actions]

Coverage

Area	main	this PR	delta
Rust (lib crates)	79.50%	79.53%	+0.03 (OK)
UI (vue/ts)	90.39%	90.61%	+0.22 (OK)

Gate: passed - no coverage regression (epsilon 0.1 pp).