V3-01: activity type per ride

Adds Activity (motorcycle/bicycle/scooter/skateboard/running/walking/other) as
a column on Trip, and threads it through as real behaviour rather than a label:
ActivityProfile (domain/activity_profile.dart) carries a noise floor, accuracy
gate, histogram bucket width, and elevation smoothing window/threshold per
activity, consumed by sanitizeSpeedKmh, isUsableFix, computeSummary, Accumulator
and speedHistogram. The motorcycle profile reproduces the exact constants the
app shipped with before this existed, and a test asserts they never drift apart.

This is the port's first real migration: schemaVersion 1 -> 2,
m.addColumn(trips, trips.activity) with a motorcycle default so every existing
row survives unmodified. Proven with a hand-built v1 SQLite file (raw sqlite3,
not drift_dev's schema tooling) that a real database with real rides upgrades
and keeps every trip, segment and point.

No picker in front of Start: a new trip defaults to whichever activity was most
recently used, derived live from the trips table rather than duplicated into a
Config field. Editable afterwards on trip detail, which recomputes aggregates
under the new profile immediately -- a walking pace that reads as noise under a
motorcycle's floor reads as real movement once the activity is corrected, and
there's a test proving exactly that transition.

The widget-test pass for the activity-picker sheet caught a real overflow bug:
seven options overflowed a Column-based bottom sheet the same way the record
screen once did. Fixed with a scrollable ListView + isScrollControlled, same
shape as that earlier fix.

188 tests (171 -> 188): 2 migration, 5 ActivityProfile, 3 profile-threading
proofs in computeSummary, 5 repository, 2 widget. Analyze clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

pubspec.lock

@@ -973,7 +973,7 @@ packages:
     source: hosted
     version: "0.7.0+eol"
   sqlite3:
-    dependency: transitive
+    dependency: "direct dev"
     description:
       name: sqlite3
       sha256: "64b2c63c8232dd20d14b34105a81ebfd74320442e8451f836179ec89986aa478"