fix(sandbox): translate the section CRUD toasts in the content browser

[pedrofrxncx]

Follows #6440/#6449, which translated the calendar and autosave toasts in the same file — this covers the section create/duplicate/rename handlers in content-browser.tsx, which were still hardcoded English.

Why: the repo's i18n rule (see CLAUDE.md) requires every user-facing string in apps/web/src to go through t(); these toasts ("Created section...", "Section not found.", "Duplicated...", "Duplicate failed", "Section renamed", "Rename failed") were shown verbatim in English regardless of the user's pt-BR preference.

Change: added 6 keys to sandbox.contentBrowser.* in both en/sandbox.ts and pt-br/sandbox.ts, and swapped the corresponding literal strings in handleCreateAvailableSection, handleDuplicateSection, and handleRenameSection for t(...) calls. No behavior change — same messages, same triggers, just localized.

Verify: cd apps/web && bunx tsc --noEmit (confirms TranslationKey completeness across both locale files) and bunx oxlint apps/web/src/components/sandbox/content/content-browser.tsx apps/web/src/i18n/en/sandbox.ts apps/web/src/i18n/pt-br/sandbox.ts.

Checks run locally: bun run fmt, bunx tsc --noEmit (apps/web workspace), bunx oxlint on the 3 changed files — all clean. No test file covers this UI-copy change; full CI validates the rest.

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Summary by cubic

Localizes section create/duplicate/rename toasts in the sandbox content browser. Previously these toasts were hardcoded in English; now they use i18n t() with new sandbox.contentBrowser.* keys so messages follow the user’s locale with no behavior change.

• Adds six keys to apps/web/src/i18n/en/sandbox.ts and apps/web/src/i18n/pt-br/sandbox.ts (includes {name} interpolation where shown).
• Replaces literal toasts in handleCreateAvailableSection, handleDuplicateSection, and handleRenameSection with t(...); keeps existing error handling (use err.message if present, otherwise translated fallback).
• No migrations or API changes.

^{Written for commit 4b5c244. Summary will update on new commits.}