docs: fix build-from-source instructions and document prerequisites

[CapnRon]

Summary

Hit these building from a stock Debian 12 (bookworm) install and want to save the next person the trial-and-error:

• The documented web build step (cd web && npm install && npm run build) skips scripts/extract_version.js, which generates web/js/version.js from CMakeLists.txt. Without it, the Vite build fails outright:

  [UNRESOLVED_IMPORT] Could not resolve '../../version.js' in js/components/preact/Header.jsx
  

  scripts/build_web_vite.sh already exists and runs that step first — the README should point at it instead of the manual npm commands.

• Three prerequisites aren't documented anywhere and surface as confusing failures partway through the build rather than an upfront "you need X":

  • FFmpeg ≥ 7.1 (libavcodec ≥ 61) — Debian 12/Ubuntu 22.04 ship older FFmpeg, and the CMake pkg_check_modules failure doesn't say what version or where to get it.
  • Node.js ≥ 20 for the web build — Debian 12's packaged nodejs is 18.x. Several deps (vite, tinykeys) require 20+/22+, and chromedriver's postinstall script crashes under Node 18 with an ERR_REQUIRE_ESM error that gives no hint the actual cause is the Node version.
  • libsqlite3-dev — not pulled in by any of the previously-listed packages; CMake configure fails with sqlite3 not found with no indication it's a missing apt package.

No functional/code changes — README only.

Test plan

• Followed the updated instructions end-to-end on a fresh Debian 12 (bookworm) install, including the FFmpeg/Node/sqlite3 prerequisites, and confirmed scripts/build_web_vite.sh → scripts/build.sh --release → scripts/install.sh succeeds where the old instructions failed at the Vite build step.

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[Copilot]

Pull request overview

Updates the build-from-source documentation to reduce common build failures on fresh Debian/Ubuntu systems by documenting missing prerequisites and pointing users to the existing web build script that performs required version extraction.

Changes:

• Adds a prerequisites list for source builds (FFmpeg versioned libs, Node.js, sqlite3 dev headers).
• Replaces manual web build steps with ./scripts/build_web_vite.sh, documenting why it’s required (generates web/js/version.js via scripts/extract_version.js).

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