test: add test project with phase 1 coverage

[jonas1307]

Test project + phase 1 coverage

Adds the first automated test coverage to the repo (xUnit), targeting the highest-risk pure logic.

Added

• Timetracker.Tests project (net10.0, IsPackable=false), joined to Timetracker.slnx. 67 tests, all green.
  • PeriodResolver — mutual exclusivity, --from > --to, invalid --period, shortcut + --from; today/yesterday/range/period success; and time-relative invariants for week (Mon–Sun, 7 days), last-week (−7d), month/last-month.
  • ValidationUtils — ValidActivityDate, ValidUrl (HTTPS enforced), TryResolveMonth (incl. leap year), ValidType (case-insensitive).
  • AddValidator, UpdateValidator, ConfigValidator — happy paths and each failing rule.
• CI — dotnet test Timetracker.slnx -c Release --no-build step between build and pack.

Deliberately out of scope (follow-up)

• Time is asserted via invariants (span/DayOfWeek), so tests don't expire; a TimeProvider seam for absolute-date tests is deferred.
• ConfigValidator's first-time-setup rules depend on ConfigService.ConfigExists() (real I/O) and are deferred until that check can be injected.

Pack of the tool is unaffected — dotnet pack Timetracker still produces only timetracker-cli.nupkg.

Closes #27