fix: third round of Docker review issues

Rust toolchain (P1):
- Dockerfile: bump builder from rust:1.87 to rust:1.88; time@0.3.51,
  time-core@0.1.9, and time-macros@0.2.30 (present in Cargo.lock) all
  require MSRV 1.88, so the real cargo build --release step was failing

single-file-cli wait mode (P2):
- singlefile.rs: replace --browser-wait-until=networkidle2 with
  networkAlmostIdle; the single-file-cli option only accepts
  InteractiveTime/networkIdle/networkAlmostIdle/load/domContentLoaded
  (verified in options.js); networkidle2 is a Puppeteer concept that the
  CLI does not recognise, causing silent fallback to the earliest state
  and incomplete captures. networkAlmostIdle is the closest equivalent
  (<=2 open connections, matching Puppeteer's networkidle2 semantics)

Build context size (P3):
- .dockerignore: add target/, frontend/node_modules/, frontend/dist/;
  these can reach 1.4G+ after a local dev build and are never read by
  the Dockerfile, so sending them to the builder wastes time and memory

Dockerfile

@@ -3,7 +3,7 @@
 ###############################################################################
 # Stage 1 – Build the Rust server and CLI binaries
 ###############################################################################
-FROM rust:1.87-slim-bookworm AS builder
+FROM rust:1.88-slim-bookworm AS builder
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
     pkg-config \