Preview PDF, DOCX, XLSX & PPTX files in the browser

[exilis]

Summary

Adds client-side preview of four binary document formats — PDF, DOCX, XLSX, and PPTX — alongside the existing markdown/mermaid/allium support. Consistent with reefdoc's "thin server, client renders everything" architecture:

• Server (Go): the tree now lists .pdf/.docx/.xlsx/.pptx (new isViewable), and /api/file sets the correct Content-Type per extension so binary bytes aren't corrupted. Path-safety is untouched.
• Frontend: a new web/viewers.js registry maps each extension to an async viewer (PDF.js → canvas, docx-preview, SheetJS, pptx-preview), each lazy-loaded from CDN on first use, so the markdown-only path pays nothing.
• Integration: one branch in show() — text files take the existing path; binary files read an ArrayBuffer (no size cap), clear the Outline, and render a static preview.

Per the design, binary documents are static previews: no TOC, no live-reload, no favorites star, no scroll-restore. Best-effort error handling.

Design: docs/specs/2026-06-25-office-pdf-viewing-design.md
Plan: docs/plans/2026-06-25-office-pdf-viewing.md

Notable detail

pptx-preview is loaded from esm.sh rather than jsdelivr — jsdelivr's +esm build of the library has a broken lodash transitive import (does not provide an export named 'get') that fails to load in the browser. esm.sh resolves it correctly with the same init named export.

Test Plan

• go test ./... — green (new tree + Content-Type/byte-integrity tests)
• npm test — 57 pass (new viewers.js registry tests)
• gofmt / go vet clean
• Browser e2e (Playwright): PDF renders to canvas; DOCX/XLSX/PPTX render real content; markdown unaffected; binary rows have no favorite star; Outline cleared on binary tabs; no console errors (besides pre-existing favicon 404)