feat(web): add routine exercise picker

[rd-code-23]

Summary

• Adds the protected /routines/new/exercises picker route.
• Connects the picker to the authenticated GET /exercises endpoint through TanStack Query.
• Validates API responses with the shared exercise contract.
• Adds local exercise-name search.
• Adds a reusable centered loading state with a spinner and label.

UI

• Uses the approved mobile-first Add Exercise layout.
• Includes close navigation, a search field, compact responsive rows, a generic exercise placeholder, subtle dividers, and row hover states.
• Includes a disabled dashed Create Custom Exercise action as a visual placeholder.
• Scales the picker appropriately for desktop without changing the compact mobile treatment.

Intentionally out of scope

• Selecting an exercise
• Configuring sets, reps, weight, rest, or notes
• Creating custom exercises
• Persisting the routine draft across routes
• Saving the routine
• Guest-mode routine storage

These remain separate follow-up slices so this PR stays reviewable.

Verification

• Lint passes
• Typecheck passes
• All 110 tests pass
• Production web build succeeds