feat: drag-drop and OS Open with for PDFs

[Matbe34]

Summary

Two related PDF entry points layered on top of the existing tabs system:

1. Drag-drop a PDF (or several) onto a running lankir window → each opens as a new tab.
2. "Open with Lankir" from the OS file manager → opens as a new tab in the existing window (single-instance default), or starts a fresh window if none is running. `Ctrl+Shift+N` spawns an explicit new window for side-by-side viewing.

App code is cross-platform (Linux/Windows/macOS); OS-side file association ships only on Linux for now (the `.desktop` file gets MIME registration). Windows and macOS need an installer / `.app` bundle that don't exist yet — flagged out of scope.

What changed

• Backend

  • `main.go` — new `parseArgs` helper splits args into "CLI subcommand" vs "GUI files to open" (Cobra-aware predicate, no extension matching).
  • `runGUI(initialFiles []string)` accepts startup files; emits them as `open-files` event in `OnStartup`.
  • `app.go` — three new methods: `onFileDrop` (filter + emit), `onSecondInstance` (resolve relative paths against the second-launch cwd + emit), `OpenInNewWindow` (`os/exec` self with `LANKIR_NEW_WINDOW=1` env signal).
  • `runGUI` wires `options.DragAndDrop`, `options.SingleInstanceLock` (skipped when `LANKIR_NEW_WINDOW=1` set), and `runtime.OnFileDrop`.
  • `cli.ExecuteWithArgs` shim added so `main.go` can pass parsed args explicitly; the existing `cli.Execute` becomes a thin wrapper for back-compat.

• Frontend

  • New `fileOpener.js` — single source funneling drag-drop / second-instance / startup files through the existing `OpenPDFByPath → createPDFTab → switchToTab` flow. Per-file errors get a toast and don't abort the batch.
  • `app.js` subscribes `fileOpener.handleOpenFiles` to the Wails `open-files` event.
  • `Ctrl+Shift+N` keyboard shortcut → `App.OpenInNewWindow(activeFilePath)`.
  • `style.css` marks `.pdf-viewer` and `.welcome-screen` as Wails drop targets via `--wails-drop-target: drop`.

• Packaging / CLI

  • `packaging/lankir.desktop` — `Exec=...lankir %f` + `MimeType=application/pdf;`.
  • `cmd/cli/root.go` — `--help` clarifies GUI dispatch and the `./pdf` workaround for files literally named after Cobra subcommands.

Tests added

• `TestParseArgs` (table-driven, 7 cases incl. `./pdf` workaround and unknown-subcommand fallback).
• `TestFilterPDFPaths`, `TestResolveSecondInstancePaths`, `TestNewWindowEnv` in `app_test.go`.
• `fileOpener.test.js` (vitest, 3 cases: single PDF, empty batch, partial backend error).

Test plan

• `go build ./...` and `go test -count=1 ./...` clean (3/3 runs).
• `task vet-cross` — Linux clean (Windows/Darwin failures are pre-existing CGO chains, unaffected).
• Frontend `./build.sh` clean; `npm test` 112/112 (pre-existing flaky perf assertion in `tests/performance.test.js` exists on master too — tracked in `TODOs.md` for separate fix).
• Manual: drag a PDF onto running window → opens as new tab.
• Manual: install .deb → "Open with Lankir" appears in Files right-click menu for PDFs.
• Manual: `Ctrl+Shift+N` spawns a separate lankir window.
• Manual: `lankir foo.pdf bar.pdf` from terminal opens both as tabs in one new window.

Out of scope (flagged in plan, deferred)

• Windows file-association registration (no installer yet).
• macOS file-association registration (no .app bundle / Wails darwin build yet).
• Tab persistence across launches.
• Drag-tab-out-to-new-window.
• Pre-existing perf-test wall-clock flake (in TODOs.md).