fix: build and verify the unpacked Linux app

[charliebarmore]

npm run package:dir failed on Linux because the fuse hook opened LedgerPDF, but electron-builder created a different executable name. The build now explicitly names the Linux binary ledgerpdf, the hook uses the packager's actual name, and package verification resolves Linux output directories instead of a Windows EXE.

The packaged smoke test now imports two PDFs, a receipt image, and a two-sheet workbook, exports all nine pages through the frozen engine, and requires nine pages after reopening the saved binder. A manual package option in the Linux workflow builds and verifies the unpacked app under Xvfb; ordinary PRs keep source-only checks. Documentation records the test scope and remaining limits.

Validation on Omarchy / Arch Linux x64, Node 26.7.0, Python 3.14.7:

• Hash-pinned packaging dependencies installed; npm run package:dir passed after reproducing and fixing the original failure.
• npm run verify:package passed: nine fuse settings, resources/licenses, frozen engine, mixed import/export/reopen, and packaged MCP with approved-folder enforcement.
• Visible UI walkthrough of the normal packaged app: placed a tick with test initials, added a note, saved, quit, reopened from Recents, and verified the tick, initials, note, nine pages, and bookmarks survived. Inspected the spreadsheet page. Used synthetic files and an isolated configuration profile.
• Release-configuration checks, workflow YAML parsing, and git diff --check passed.

The walkthrough used agent-browser against the normal packaged window, not a human acceptance test. Native file dialogs, installers/desktop registration, ARM builds, and portability of Arch-built binaries remain unverified. Details: docs/LINUX-VERIFICATION.md.

Follow-up to #20 and #21.

Clean-runner validation: Linux packaging run passed on Ubuntu 22.04 with Node 22 and Python 3.12 at commit 44aabd3, including nine fuse settings, frozen engine, nine-page mixed import/export/reopen, and packaged MCP. All PR source checks on Linux, Windows, and macOS plus CodeQL passed.