fix(ui): resolve fence language from the filetype, not the extension - #539

Problem

Code fences in the output and permission dialog are labelled with a string that
is not always a treesitter language name, so some files never get syntax
highlighting. .rs was labelled rs (not rust), .yaml became yml, etc.

Root cause

vim.filetype.match (Neovim core) already resolves almost every path to a
filetype. Neovim then resolves a fence's info string with has_parser(name) /
get_lang(name), where get_lang maps a filetype to a language — not an
extension.

The two code paths had also diverged:

• The formatter already reused the helper.
• The permission dialog was written separately and derived the fence language
  from the raw extension (fnamemodify(path, ':e')), so .rs became rs,
  which no parser resolves.

The helper itself used a hand-written override table that was incomplete and
partly wrong, and duplicated what get_lang already does.

Fix

• permission_window: reuse the helper instead of fnamemodify(path, ':e').
• util.get_markdown_language (renamed from get_markdown_filetype): resolve
  path -> vim.filetype.match -> vim.treesitter.language.get_lang, falling
  back to the extension.
• The context/formatter callers use the renamed function.

The old table was incomplete (only a handful of the filetype != language
pairs) and wrong:

• yaml -> yml: no parser, and it turned a case that resolved on its own
  (yaml) into a broken one.
• javascipt -> js: a typo that never matched.
• typescript -> ts, javascriptreact -> jsx: only worked because
  nvim-treesitter aliases those extensions.
• text -> txt: not a language.

Why the small table exists

get_lang only maps filetypes to languages when nvim-treesitter is
installed — Neovim core registers almost nothing. The plugin keeps a small
table for the common name mismatches so setups without nvim-treesitter still
highlight them. The two overlap and complement each other:

Verify by listing every filetype whose name is not its language:

for _, ft in ipairs(vim.fn.getcompletion('', 'filetype')) do
  local lang = vim.treesitter.language.get_lang(ft)
  if lang ~= ft then print(ft, '->', lang) end
end

• Without nvim-treesitter: prints nothing — every filetype falls back to
  itself, so sh, cs, javascriptreact, ... have no language and would not
  highlight without the table.
• With nvim-treesitter: 43 entries, e.g. sh -> bash, cs -> c_sharp,
  svg -> xml, javascriptreact -> javascript, pandoc -> markdown.

Verification

• New unit cases for the path -> language mapping: rs -> rust, sh -> bash,
  svg -> xml, tsx, jsx, empty filename, unknown extension.
• Full suite green (minimal / unit / replay). Four replay snapshots were
  regenerated because the fence label changed (txt -> text).