history: one workoutAt(w), replacing the copies that had drifted

[kurktchiev]

"When did this workout happen" was written by hand ten times in eight files, each as
w.start || new Date(w.d).getTime() or a close variant: lib/coach-insights.js,
lib/effort.js, lib/muscles.js, lib/recovery.js, lib/strength-exercises.js
(two copies), lib/coach-demo.js, views/Stats.jsx (two copies), and the MCP
server's muscle_balance in mcp/src/tools.js.

new Date('2026-09-22') is UTC midnight, which any negative UTC offset drags
back into the day before. Every other date helper in the app parses local
noon so no timezone or DST shift can move the day. And ||
discards a legitimate start of 0.

lastWorkSetAt and the effort/recovery windows feed the retained-strength
and fatigue decay off this, so imported history with no start was dated at
UTC midnight instead of local noon: up to a day early west of UTC (17 h at
UTC-5), and nothing reported it. muscleBalanceWindow and the Stats volume/last-trained
numbers dated their own cutoffs the same way, which can move a workout in or
out of a window right at the edge.

One exported workoutAt in history.js now answers it, and every call site
routes through it, the MCP server included (it already imports history.js),
so there's one rule to get right. exercise-history.js
already had this right in its own local startOf; it now shares the same
function instead of keeping a correct copy of its own next to the wrong ones.
entriesWithId in strength-exercises.js keeps -Infinity for the undated
case on purpose, because isNewerOccurrence compares with !== then >,
and a NaN pair would answer "not equal, not greater" instead of falling
through to insertion order.

frontend 9 new tests in workout-at.test.js, run with npx vitest run --maxWorkers=2, including a source-tree grep that fails if any file outside
history.js, in frontend/src or mcp/src, hand-rolls the pattern again.
7 of the 9 fail against the old code (checked by reverting the fix and
re-running), and the grep test names all eight offending files. With the fix
in, the full frontend, mcp and api suites pass on 39bc71e.

Conflicts with two open PRs, beyond the ones each already has with main:

• feat(engine): generic prescription engine v2, smart warm-ups and v1→v2 migration #312 (generic prescription engine v2) in coach-demo.js,
  coach-insights.js and exercise-history.js.
• Sets-based fatigue with per-muscle recovery, RIR quality and estimated effort #436 (sets-based fatigue with per-muscle recovery) in recovery.js.

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