fix: raise Next.js proxy body buffer so large uploads work

[BrianBoyCN]

Problem

Uploading a file larger than 10 MB through the Pi Web file panel fails with HTTP 500:

{"error":"Failed to parse body as FormData."}

Smaller uploads work. The failure is deterministic at exactly the 10 MB boundary, and it is not a proxy problem — it reproduces on 127.0.0.1:3000 with no reverse proxy in the path.

Root cause

Pi Web registers proxy.ts with matcher: ["/", "/login", "/api/:path*"], so every API request passes through the Next.js proxy/middleware layer. When a proxy is present, Next.js clones and buffers the request body and caps that buffer at 10 MB by default (experimental.proxyClientMaxBodySize).

Once the body exceeds the cap, Next only forwards the first 10 MB. The upload route's parseFormDataWithinLimit() then rebuilds a Response from the truncated bytes, and undici's multipart parser throws Failed to parse body as FormData. — which the route surfaces as a 500.

The server log shows the real reason:

Request body exceeded 10MB for /api/files/.../route?type=upload&conflict=error.
Only the first 10MB will be available unless configured.

The upload route advertises a 25 MB per-file / 100 MB per-request limit with dedicated 413 responses, but requests never reach that logic — the proxy layer truncates them first.

Fix

Set experimental.proxyClientMaxBodySize above the route's own 100 MB cap, configurable via a new PI_WEB_MAX_BODY_SIZE environment variable (default 128mb):

// next.config.ts
experimental: {
  proxyClientMaxBodySize: resolveMaxBodySize(),
},

• bin/max-body-size.mjs parses b/kb/mb/gb suffixes or a raw byte count, falling back to 128mb for blank/invalid input.
• The value is read when next.config.ts loads, so it can be changed without rebuilding (verified below).
• README env tables updated in all four languages, and pi-web --help lists the new variable.

Verification

Built the app, then against the production server:

Upload	Before	After
5 MB	200	200
20 MB	500 Failed to parse body as FormData.	200
26 MB	500	413 Each upload must be 25MB or smaller (route's own limit)

Runtime override confirmed by starting the built server with PI_WEB_MAX_BODY_SIZE=1mb: a 5 MB upload then fails with the 1 MB buffering warning, proving the env var takes effect without a rebuild.

npm test (1024 passing), npx tsc --noEmit, and npm run lint are all green. New unit tests cover size parsing/fallback and assert the config stays above the 100 MB upload cap.

Fixes the missing 10 MB buffering configuration introduced when proxy.ts was added.