Add routine exercise configuration shell

[rd-code-23]

Summary

Adds the next routine-creation step: selecting an exercise now opens a dedicated configuration screen based on the approved desktop and mobile mockups.

What changed

• Makes every exercise-picker row navigate to its selected exercise configuration URL.
• Adds a sibling TanStack Router route for /routines/new/exercises/$exerciseId, allowing the configuration page to replace the picker instead of rendering inside it.
• Loads the selected exercise from the existing exercises query and handles loading, error, and not-found states.
• Builds the responsive configuration shell:
  • mobile uses a stacked layout;
  • desktop uses a 25/75 split between exercise identity and controls;
  • the custom-only Edit name action is shown only for custom exercises.
• Adds a reusable number stepper with editable keyboard input plus minus and plus controls.
• Adds a reusable positive-integer input for weight and rest.
• Prevents negative, decimal, and non-numeric values in the current configuration controls and restores the minimum value when an empty field loses focus.
• Keeps related labels and controls aligned on the same row and uses compact segmented controls that match the mockups.
• Applies the documented Tailwind readability convention by grouping longer class lists with cn().

Current scope

This PR establishes the configuration UI shell and local input behavior. Saving the selected exercise into the routine draft, custom exercise name editing, progression, and final form submission remain later slices.

Verification

• Full web test suite: 35 tests passed
• Web lint passed
• Web type-check passed
• Production web build passed