feat(installer): wizard-style layout (banner + status + buttons footer)

[nelsonduarte]

Summary

Restyle the installer to look closer to LibreOffice's wizard-style installer (user shared a screenshot of LibreOffice 26.2.3.2 - Assistente de instalação and asked for "algo parecido").

Layout changes

• Drop the old dark-blue full-bleed header (#1E3A5F with white "PDFApps" brand text). The brand block reads as decoration on a wizard window and steals vertical space we'd rather give to the form.
• New top banner: 80 px light blue-gray band (#E5EDF5) with a bold title ("Install/Instalar PDFApps {ver}") and a one-line subtitle inviting the user to choose options. 1 px border line separates banner from body.
• New footer: separated from the body by a border line, contains a bold Status: / Estado: label, the current-operation StringVar (was previously inside the body), and the determinate progress bar. Action buttons (Install + Cancel) pack right-aligned below the progress bar, mirroring LibreOffice's Anterior / Seguinte / Cancelar row.
• Window grew from 520×470 to 600×520 to give the form fields + new footer room to breathe.

Files

• installer.py — _build() rewritten; 3 new i18n keys × 8 languages (banner_title, banner_subtitle, status_label).

What's NOT changed

Form fields, checkboxes, install logic, the threaded _install worker, the splash sequence, and the platform-specific shortcut / Tesseract / Ghostscript install paths. This is a pure layout / styling refactor of _build().

Test plan

• Syntax check on Py3.14: ast.parse(...) clean.
• UI builds without errors under xvfb on Ubuntu 26.04 + Py3.14: 600×520 window, 7 frames in the expected packing order (banner → top border → body → above-footer border → progress area → buttons row → bottom border) plus the loading splash Toplevel.
• Visual check on Windows: run py installer.py from source — confirm the banner renders, body still has folder + checkboxes, progress bar visible at bottom, Install + Cancel buttons right-aligned.
• Re-run a full install end-to-end (Tesseract + Ghostscript download paths) — none of that logic changed but the status/progress widgets moved, want to confirm _step() still updates them correctly.

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