docs(uploads): four-mode upload recipes + nav (#67)

[adnaan]

Completes 3 of the 5 tasks in #67 — the docs-native recipes + nav + the go.mod bump. The other two (rewrite content/reference/uploads.md, run cmd/sync) are mirrored content owned by the sync pipeline and landed via #72 (livetemplate@v0.13.0) + #73 (client@v0.13.0), both now merged. This PR deliberately does not touch content/reference/uploads.md.

Do not auto-close #67 on merge — see "Remaining for #67" below. (Commit references (#67), not Closes.)

Changes

• go.mod — bump livetemplate v0.12.0 → v0.13.0 (the release that ships the four upload modes), matching the reference doc's source_ref.
• recipes/apps/avatar-upload.md — rewritten to mirror the (already-modern) examples/avatar-upload/ code, framed as the default Volume mode. Drops the long-removed AllowUploads/ConsumeUpload API, hardcoded /Users paths, and v0.3.0 references.
• recipes/apps/upload-modes.md — new recipe walking all four modes (Volume / Direct / Proxied / Preview), pointing at examples/upload-modes/.
• recipes/ui-patterns/forms/file-upload.md — names the default Volume mode and links the four-mode reference + new recipe.
• tinkerdown.yaml + recipes/apps/index.md — register the Upload Modes page.

Verification

• tinkerdown validate content/ — 94/94, 0 errors; the three touched/added pages validate.
• cd cmd/sync && go test ./... and GOWORK=off go build ./... + go mod tidy — clean.
• Built the production Docker stack and ran the chromedp e2e suite against it: IA / breadcrumb / recipe-nav / responsive / patterns all pass; both new pages render correctly in-browser at desktop + mobile viewports with the nav entry present.
• The 4 TestSpine* failures seen locally are environmental (server-push/WebSocket through a local Docker bridge), not from this change: the v0.13.0 diff is upload-only, main's CI e2e is green, and a pristine v0.12.0 image fails the same 4 identically.

Remaining for #67 (follow-up, not in this PR)

The upload-modes example's browser E2E is gated behind LVT_UPLOAD_MODES_E2E + LVT_LOCAL_CLIENT (unset in CI), and its skip message cites docs#67 as the issue to "integrate it into the CI harness." Docs CI runs e2e/... only, never examples/..., so that test currently runs nowhere. Wiring it in needs the client bundle + a CI job — suggest a dedicated follow-up issue rather than expanding this PR.

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