handle missing gracefully

[andiwand]

collect() — missing files are now comparable: True, so they render as clickable links into the compare view (keeping their missing in reference (A) / missing in monitored (B) message and different status). Missing directories stay non-linkable, since there's no single file to open.

/file/<variant>/<path> — when the file is absent on the requested side, it serves a small centered "file missing in reference (A)/monitored (B)" placeholder page instead of a raw 404, so the iframe shows something meaningful while the other side renders normally.

/image_diff — returns 404 when either side is missing (nothing to diff), so the diff strip's just doesn't render instead of crashing on to_url's FileNotFoundError.

So now for a missing file you get:

• The existing side viewable in the compare view, the other side showing a "missing" placeholder.
• The ▶ / update ref button working for files present in monitored-but-not-reference (copies B→A, creating it). For the reverse (present in reference, missing in monitored) update-ref correctly 404s, since there's no monitored source to copy.

/update_ref needed no change — it already copied B→A and 404'd on a missing source. All 3 tests still pass.

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Added a collect_one_sided() helper in root() that walks a directory present on only one side and emits one row per comparable file inside it (recursing into nested subdirectories), each marked missing in monitored (B) / missing in reference (A) with a different status. The old single opaque "directory missing" row is gone.

So if only_a_dir/ exists only in the reference, you now get:

• only_a_dir/p.html → linked, "missing in monitored (B)"
• only_a_dir/nested/q.html → linked, "missing in monitored (B)"

Each row behaves like any other missing file:

• Viewable in the compare view (present side renders, other side shows the "file missing" placeholder).
• Copyable via update-ref where a monitored source exists (B→A), creating the file/dirs as needed (shutil.copy2 plus update_ref's existing copytree path handle parent creation).

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blocked by #12