Cultures + Lieux: delete varieties and locations (guarded)

[guycorbaz]

Continued pre-production feedback: a variety couldn't be removed, and a location couldn't be deleted — both screens lacked the affordance (same gap as crops had).

Changes

• Variety delete (Cultures screen): each variety row gets a Supprimer button, routed through the shared confirmation dialog (Confirmations de suppression (dialogue partagé) #61). Blocked — with a localized message — when a planting uses the variety (planting.variety_id is ON DELETE RESTRICT). delete_variety + VarietyRow.in_use disables the button.
• Location delete (Lieux screen): each location row gets a Supprimer button. Blocked when the location holds child locations or plantings (both FKs are RESTRICT) — exactly as requested: in_use disables the button and delete_location refuses with a localized message. LocationListItem.in_use is computed from children + plantings.
• New i18n keys (fr/en) for the confirm + in-use/refusal messages.

Verification

• App tests: delete_variety (unplanted → ok, planted → refused), delete_location (unused leaf → ok, has-children → refused, has-planting → refused).
• cargo test --workspace (335) green, cargo clippy --workspace --all-targets + cargo fmt --check clean.

Note: crop/location editing of arbitrary fields beyond what's already shipped (crop edit landed in #88) can follow if needed; this PR focuses on the deletion gaps reported.

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