id(web): detect_mode's .pm.json check is case-sensitive — document.PM.JSON opens Raw instead of Rich (and downloads as doc.PM.JSON.pm.json) #770

Summary

detect_mode treats every extension case-insensitively except the compound
.pm.json extension: a file named document.PM.JSON (or Document.Pm.Json)
opens in Raw mode instead of the Rich (ProseMirror) editor, and creating it
seeds empty bytes instead of the empty PM document {}.

Evidence

pkgs/id/src/web/content_mode.rs:143-152 — the extension is lowercased before
matching, but the Rich guard compares the raw filename:

let extension = path.extension().and_then(|e| e.to_str()).map(str::to_lowercase);
match extension.as_deref() {
    Some("json") if filename.ends_with(".pm.json") => ContentMode::Rich,

ends_with is case-sensitive, so the guard fails for any non-lowercase
spelling even though the json arm matched. Every other arm is effectively
case-insensitive, and the module's own tests pin that contract for simple
extensions (test_detect_markdown asserts CHANGELOG.markdown → Markdown).

Verified this run with a standalone rustc reproduction of the exact guard
(Path::extension + to_lowercase + ends_with, std semantics):

document.pm.json: rich=true
document.PM.JSON: rich=false
document.Pm.Json: rich=false
notes.MD ext lowercased: Some("md")   // simple extensions ARE case-insensitive

Consequences on current main (15f4885):

• pkgs/id/src/web/routes.rs:1227 (new_file_handler): creating
  notes.PM.JSON takes the Raw content branch (b"") instead of Rich
  (b"{}").
• Opening the file routes through detect_mode_with_content
  (routes.rs:452, collab.rs:121) → Raw, so a ProseMirror document is shown
  as raw text and edited without the rich editor.

Second site with the same root cause: pkgs/id/src/web/routes.rs:1676
(download-as-JSON name) — req.name.ends_with(".pm.json") is case-sensitive,
so downloading doc.PM.JSON as JSON names the download
doc.PM.JSON.pm.json.

Precedent in the same codebase: src/world_ssh.rs:985 lowercases before
ends_with(".md") checks.

Why it matters

Filenames with uppercase extensions are common from Windows/macOS exports
and bulk renames. The mode detector's contract is "extensions are
case-insensitive" everywhere else; the one compound extension silently
violates it, and the file then opens in the wrong editor with the wrong
seed content.

Acceptance criteria

• detect_mode returns ContentMode::Rich for .pm.json in any case
  (document.PM.JSON, Document.Pm.Json, …), with regression tests next
  to test_detect_prosemirror_json in content_mode.rs.
• Simple-extension behavior unchanged; the existing content_mode tests pass.
• The download-as-JSON path (routes.rs:1676) treats an existing .pm.json
  suffix case-insensitively too, or the issue is updated noting why that
  site was deliberately left.