Add a by-session view with dispersion ellipses and hover isolation

[chayuto]

What

• New By club / By session toggle in the filter bar. Session mode colours dots on a sequential ramp, oldest cold to newest warm, so progression reads as a colour sweep.
• Toggling By session narrows to the busiest club. Session comparison across mixed clubs is noise. The previous club selection is restored on the way back unless changed in between.
• Dispersion ellipses are now rotated covariance ellipses at 2 sigma, oriented along the shot pattern, in both the fan and the shot shape chart. They fit full swings only: a variance-minimising two-cluster split drops drills and chips from the fit while their dots stay plotted.
• Interactive date legends under both charts. Hovering a date or a dot isolates that session and dims the rest.
• The shot shape chart links session means with a dashed trail.
• The trend card draws faint per-session mean +/- 1 sigma boxes behind the session average dashes.

Fixes

• StrictMode double-mounts the initial load. The abandoned duplicate fetch could resolve after a user edit and overwrite it with stale data. Loads are now cancelled on effect cleanup.
• A failed stats refetch after excluding a shot reverted the dot even though the flag had persisted. The revert now only happens when the PATCH itself fails.

Testing

• New e2e test covers the grouping toggle, session legends, and hover dimming. Full local run: 15 passed.
• Ellipse and cluster-split maths verified numerically (centre, radii, orientation recovery, 86% coverage at 2 sigma).
• rspec, rubocop, brakeman, bundler-audit, npm lint, npm build all green.