chore(usage): remove unused fetchUsage()

[dobby-coder]

Removes fetchUsage() from src/lib/usage.ts. It performed a pre-flight GET /usage?email=... against the cryptify backend but was never called — a tree-wide search finds only its definition. The live usage query was added in #110 (2026-04-23) and never wired into the UI; sender quotas are enforced by the server 413 backstop (parseLimitExceededBody) plus the browser-local estimate in src/lib/localUsage.ts.

• Deleted fetchUsage().
• Dropped the now-unused FILEHOST_URL from the usage.ts import (ROLLING_LIMIT stays).
• classifyUsage, UsageResponse and UsageStatus are untouched — still reachable through parseLimitExceededBody.
• Refreshed the CLAUDE.md note that described the never-built UsageWarning/fetchUsage path so the doc matches the tree.

No behavioural test applies: this only removes unreferenced code, and svelte-check would fail if anything still referenced the symbol. Verified locally with npm run check, npm run lint, npm run test:unit, and npm run build — all pass. Same pattern as the recently merged #326 (removing unused GetBrowserInfo()).

Closes #323

[dobby-coder]

VERDICT: approve

Rules + review sign-off, cycle 1. No blocking findings.

Checked against main: fetchUsage has no callers (only its definition), and FILEHOST_URL stays in use elsewhere through its own imports, so dropping it from the usage.ts import line is safe. classifyUsage, UsageResponse and UsageStatus remain reachable through parseLimitExceededBody. The CLAUDE.md note now matches the tree. CI is green across the board, the title passes the Conventional Commit check, a reviewer is assigned, and Closes #323 is registered as a closing reference.

One rule-check candidate (PR body length vs the no-justification-paragraphs rule) was dismissed: issue #323 explicitly asked for a product call between removing and wiring up the function, so the body documenting why removal is safe is warranted here.