Fix non-ASCII and special-character filenames in directory mode - #122

Problem

Directory-listing mode (-D) had two bugs that broke non-ASCII and special-character filenames:

1. Double percent-encoding in live reload (script.js)
   window.location.pathname is already percent-encoded by the browser
   (e.g. /日本語.md → /%E6%97%A5%E6%9C%AC%E8%AA%9E.md). The
   loadMarkdown() fetch wrapped it in encodeURIComponent() again,
   producing %25E3%2583%2586…. The /__/md endpoint decoded only one
   layer, looked up the literal string %E6%97%A5… on disk, and returned
   404. The correctly server-rendered page was then overwritten with
   an error message.

2. No HTML/URL escaping in the template (text/template)
   The HTML template used text/template, which performs no contextual
   escaping. Filenames containing &, <, > could break the markup,
   and filenames containing # or non-ASCII characters produced invalid
   href attributes (e.g. href="/file#1.md" — the browser treats
   #1.md as a fragment).

Changes

internal/server/static/script.js

Decode window.location.pathname before re-encoding so the /__/md
query parameter is single-encoded:

// before
`/__/md?path=${encodeURIComponent(window.location.pathname.slice(1))}`
// after
`/__/md?path=${encodeURIComponent(decodeURIComponent(window.location.pathname.slice(1)))}`

internal/server/server.go, types.go, directory_handler.go

• Switch text/template → html/template for automatic contextual
  HTML escaping of filenames, titles, and breadcrumb labels.
• Add a urlPathEscape template function that percent-encodes each
  path segment (url.PathEscape) while preserving / separators.
• Change TemplateParam.Body and HeadingsHTML to template.HTML
  so intentionally rendered Markdown HTML is not escaped.

internal/server/template.html

Use {{urlPathEscape .Path}} in all href attributes (file tree,
directory index, breadcrumbs, back-to-README link).

Tests (internal/server/nonascii_test.go, fixtures in testdata/nonascii/)

• /__/md endpoint returns correct content for Japanese/Chinese paths.
• Page render returns 200 for non-ASCII, &, and # filenames.
• Directory listing HTML-escapes display names (foo&bar.md).
• Directory listing percent-encodes hrefs (%E6%97%A5…, file%231.md).
• Rendered Markdown body is not double-escaped.
• Breadcrumb labels render correctly.

Affected flags / modes

• --directory-listing (-D): primary fix target.
• Single-file mode: unaffected (no path in URL, no directory listing).
• No flag interface changes.

Testing

GOCACHE=/tmp/go-build CGO_ENABLED=0 go test ./internal/...
./jslint.sh internal/server/static/script.js