Organize routines frontend feature files

[rd-code-23]

Summary

Reorganizes the routines frontend feature into smaller folders grouped by responsibility. This is a file-organization refactor only; application behavior and URLs are unchanged.

New structure

• api: routines fetcher, TanStack Query configuration, and API tests
• landing: routines landing screen and its tests
• create: shared creation-flow state and screens
• create/editor: routine editor, schedule field, and editor tests
• create/exercise-picker: exercise picker and its tests
• create/exercise-configuration: exercise configuration and its tests

Cross-feature imports use the existing @/ source alias so moving files deeper does not introduce hard-to-read ../../../../ imports. Tests remain beside the files they cover.

Dependency

This PR is currently stacked on PR #80 because it moves files introduced or changed there. Review this PR as the single folder-organization commit; after PR #80 merges, its base can be changed to main.

Verification

• web lint
• web type checking
• all 37 web tests