feat(data): multi-exercise support with full backward compatibility

[Copilot]

Adds data layer support for tracking multiple exercises per day using a semicolon-delimited CSV format, while remaining fully backward-compatible with existing single-exercise entries and CSV files.

Data format

date,weight,calories,steps,exerciseType,exerciseMinutes
2026-05-15,78.4,2100,8450,Cycling;Walking;Strength,60;45;20
2026-05-14,78.2,2150,9200,Running,40

Internal IndexedDB representation adds an exercises array alongside the existing legacy scalar fields (populated from exercises[0] for UI compatibility):

{
  "exercises": [
    { "type": "Cycling", "minutes": "60" },
    { "type": "Walking", "minutes": "45" }
  ],
  "exerciseType": "Cycling",
  "exerciseMinutes": "60"
}

Changes

src/lib/db.js

• parseExercises(typeString, minutesString) — splits semicolon-delimited CSV strings into { type, minutes }[]; pairs by index then filters empty types, so sparse entries like Cycling;;Strength,60;;20 are handled correctly
• serializeExercises(exercises) — converts array back to { types, minutes } delimited strings
• sanitizeEntry — normalizes old entries (no exercises field → built from legacy scalars) and new entries (uses array directly); always back-fills exerciseType/exerciseMinutes from exercises[0]
• createEmptyEntryDraft — includes exercises: []
• isEntryEmpty — extended to check exercises array

src/lib/daily-log-csv.js

• parseEntriesCsv — parses both Running,40 and Cycling;Walking,60;45; normalizes exerciseType/exerciseMinutes to first exercise
• serializeEntriesToCsv — serializes from exercises array; single-exercise output is unchanged
• createDailyLogCsvTemplate — updated to show multi-exercise example row

src/components/app/DailyHistoryPage.helpers.js

• formatExerciseSummary — renders Cycling • 60 min + Walking • 45 min; falls back to legacy scalar fields for old entries
• hasExercise — checks exercises array first, then legacy fields

Tests

• 33 new tests covering parseExercises/serializeExercises edge cases, multi-exercise CSV parsing, old-format backward compat, round-trip serialization, and display helpers
• Fixtures updated to include exercises field