Full parity between built-in and custom exercises

[giulioleuci]

Summary

Closes #199.

Built-in exercises become user-overridable and hideable without copying the 1,324-entry catalogue into user state; custom exercises gain the structured fields built-ins already had (steps, media). Both kinds now share one editor and one safe hide/delete flow.

• Central resolver (lib/exercises.js): two new state fields, exOverrides and deletedEx, overlay onto the pristine CATALOGUE/EXDB, which are never mutated. Every listing surface (library, pickers, search) resolves through this one place, so an edit or hide shows up everywhere at once. Search now indexes exercise steps; imgSrc/gifSrc pass through absolute URLs for custom media.
• Unified editor (sheets.jsx): the existing custom-exercise form now edits either a custom exercise or a built-in override — identical fields for both (name, body part, equipment, primary/secondary muscles, description, steps, media), plus a per-field "reset to default" control shown only where a built-in's value has actually been overridden. Saving a built-in diffs against its catalogue row and persists only the touched fields, dropping the override entirely once it matches the catalogue again. Delete/Hide share the active-workout guard and routine/history cleanup — customs delete permanently, built-ins hide reversibly via deletedEx.
• Settings: a "Manage hidden exercises" screen lists every hidden built-in with per-item Restore and Restore all.
• Fixed along the way: hidden built-ins stay resolvable by id (with any override applied) so historical workouts, recovery, muscle balance and stats keep rendering them correctly instead of falling back to a raw id or placeholder; i18n instruction/name packs no longer silently shadow a user's override for non-English languages; the catalogue's own duplicate names no longer block editing a built-in whose name is untouched.

Test plan

• cd frontend && npm test — 107 files / 1486 tests passing
• cd frontend && npm run build — clean production build
• Manual trace of cross-cutting scenarios: hide + override on the same built-in, restore preserving the override, active-workout guard on both exercise kinds, non-English steps/name override precedence over the generated i18n packs

🤖 Generated with Claude Code

[giulioleuci]

Implement #199

[DuarteSantos8]

Thanks @giulioleuci. v1.3.9 took one part of this area: custom exercises can carry a photo, GIF or short video and a link, kept on the server and the device, outside the synced data. The override/hide model for built-in exercises (exOverrides/deletedEx) and media on built-ins wait for the catalogue milestone (v1.4.6), so they can be designed together.

When you get back to it, please rebase onto v1.3.9, reuse the new media store for custom media, and leave out the docs/superpowers files.