refactor: deduplicate build scripts and fix fragile patterns

[andreabadesso]

Problem

PyInstaller entry scripts duplicated 3x

The same entry script and runtime hook are copy-pasted in:

• `scripts/build-hathor-core.sh` (lines 62-122)
• `scripts/build-tx-mining-service.sh` (lines 75-126)
• `scripts/windows/build-hathor-core.ps1` (lines 40-79)

Target triple detection duplicated

Same `if [[ "$OSTYPE" == "darwin"* ]]` block in 3 scripts.

tx-mining-service deps hardcoded

`scripts/build-tx-mining-service.sh:56-62` manually lists pip dependencies instead of using the project's lock file. Will silently diverge.

wallet-headless runs npm install twice

`scripts/build-wallet-headless.sh:57-58` runs `npm install` twice — second call re-downloads everything.

build-explorer uses npm install instead of npm ci

Non-deterministic; can update lock file.

Proposed Solution

• Extract shared PyInstaller template to `scripts/lib/pyinstaller-entry.py`
• Extract target triple detection to `scripts/lib/detect-target.sh`
• Use `pip install -r requirements.txt` or poetry export for tx-mining-service
• Fix wallet-headless to use `npm run postinstall` instead of second `npm install`
• Use `npm ci` in explorer build